Before you get into this guide, I'd like to clear up a couple things:

• This guide/build is for 3v3. 5v5 requires a completely different playstyle and build.
• This won't make you God. Yes, it should help you, but you still need to use common sense. And remember, practice makes perfect.
• Nocturne is so bad-*** he's probably going to be nerfed.
• This build gives you some survivability, but you will still pretty much fold like paper if you get focused. You want your enemies to turn and run as soon as you attack them, so you can just keep killing them.

Now, you're wondering, why would you want Nocturne in 3v3? I mean, 3v3 is tanky-dps heaven, right?
And yes, you would be right. Nocturne doesn't exactly fit in. The real reason Nocturne is so strong in 3v3 is his ultimate, Paranoia. At level 6, he gains a dash from the length of your top turret to your enemies top turret - Around half the map. Never again will you see Lux getting away with 150 hp. Thats before you even get to the amazing gankability he has on TT. He can get almost anywhere on the map within about 15 seconds, either to gank or to support a team mate. A bit like Shen can help any team-mate, just with a smaller range.
However, Nocturne needs good team-mates. Any tanky characters with CC, such as Shen or Rammus are good partners, or even Xin Zhao. As you should be playing on TT, this shouldn't be a huge issue, but if you're playing with two other paperlike squishies, you're going to have problems.

I find that summoner spells are far more important in 3v3 than 5v5, especially for firstblood. Of course, you should take what you find works, but these are what I like:

Exhaust - Huge slow (70%), decent damage decrease. Perfect for first blood, great throughout the game. No brainer for me. Particularly effective against DPS characters, this helps chase, escape, and gank. Perfect for Nocturne, as he has no hard CC.
Heal - I never usually consider taking this for a champion, for me it used to be the stuff of noobs, or low ELO. And yes, it does get used by new players a lot. But just because that is true doesn't make it bad. The same goes for Master Yi. In low ELO he is a steamroller, but high ELO he is one of the hardest characters to play because of how easy to counter he is.
I like taking Heal, as it gives me the edge in teamfights that Nocturne needs. Without this, even once you get Frozen Mallet you will still die really quickly. I can't emphasise this enough. You need stuff to keep you safe, this is one of them. Not to mention it's a great team skill - every team should have one, just for emergencies. This can turn the tide in a teamfight.
Personally, I like using those. However, there are still other viable ones according to personal choice.
Flash can be good, since it's 3v3. I personally don't like flash, I find the cooldown too long, and the utility rather little, but take it if you want.
The other good one for Nocturne is Ghost - Again, I don't use Ghost for Nocturne, as I find he already has good chasing abilities. Generally, you should replace Heal with these.

I personally take a mixture of tank and AD carry runes for Nocturne, to increase his damage output and his survivability.
Fairly obviously, Greater Mark of Desolation is a good choice for Nocturne, increases the damage on his abilities and autoattacks. With runes, Brutaliser and masteries, you end up with 30 ArP earlygame, and since most champions have no more than around 70 late game, that cuts out about half of their armour - even more earlygame.
For Seals and Glyphs, I go tanky to help with Nocturne's paper-like lane phase. Once you get Phage you won't be so squishy, but early-game you need the boost from Greater Seal of Armor and Greater Glyph of Magic Resist, especially when going for first blood.

The most unusual part of my rune page for Nocturne are my Greater Quintessence of Movement Speed. Very few people use these for any champion, so not many people have bought them. If that's the case, then replace them with whatever you have - Greater Quintessence of Health works well, or Greater Quintessence of Desolation. However, I find movespeed to be the most useful quintessences. It allows you to chase enemies better, especially when on Dusk Trail:Hastened, as well as escaping or travelling. Overall, very good on such a mobility oriented champion, but can be replaced by others.

I again focus on giving Nocturne's earlygame a bit of survivability by taking 21 points in defence. I also take 9 in Offence to give him a bit more ArP, but those could also be put in utility.

OK, items. The big one :)
All the stuff up to this point won't make a huge difference beyond the first 5 minutes.
Items are what changes the whole game. As with all my guides, I want to emphasise this isn't a clearcut build, there are better and worse items for each slow depending on your team and your enemies team. Build situationally.


I originally used Doran's Shield as a starting item for Nocturne. In 5v5, it's far superior to Doran's Blade, but blade works well for 3v3. You do need to leave your lane often, but your high movespeed and the short distances mean that you won't lose much xp/gold. Furthermore, Doran's Blade gives you some damage earlygame, which is what we've been lacking up till now. Combined with Umbra Blades and the 20 damage bonus from Dusk Bringer, you hit like a ton of bricks even at level 1. I've played a total of about 11 games with this as a starting item so far. I HAVE GOT FIRST BLOOD IN EVERY. SINGLE. ONE, whereas before (with Doran's Shield), my damage was a bit lackluster earlygame.


If you DID get First Blood, then you should have enough to buy your Boots of Speed]. Generally, I recall to buy these and take top lane. If you're team was lucky enough to get two or three kills, then buy [[Boots of Speed and a Dagger to build into your Berserker's Greaves. I then put the remaining cash into pots, usually 2-3 Health Potions and 1-2 Mana Potions. Since you only need 150 gold to upgrade to Berserker's Greaves at this point, if you get a kill or two, and have around 400+ gold, it's worth moving on to the next part of the build and coming back to the boots when you have less money.


The earlier in the game you get this, the better. This is what makes your attacks hit so hard earlygame. It gives you a decent amount of damage for a low price, but the armour penetration it gives you is the star of the show. Early-game, unless your enemies specifically buy armour items, this should negate around 80% of their armour.


Adds yet more damage, and a little bit of health. (You really should've bought your Berserker's Greaves by the way.)
The slow is a nice touch, but doesn't do much yet. If you're really really roflstomping the enemy team, then it's worth skipping finishing off Frozen Mallet and buying a B. F. Sword first.


This works wonders with Nocturne's chasing abilities. He's already a great chaser, but once you have this there is no escape. Once you've got this you should also be able to survive most situations, within reason. (Don't just sit there while 3 of them are attacking you. Yes, I've seen that done.)
I occasionally see a Nocturne rush this item, but it is useless without damage.Remember that, get The Brutalizer first.


Once you have your Frozen Mallet, you once again need more damage. Keep in mind that yes, you can buy tank items, but you won't do any decent damage. These two provide some of the best stats-for-gold ratios in the game (THE best for damage output) and build into great items later on. Just get these when you have the cash for them, you should be able to get kills anyway.


The same applies to these. They both work well for Nocturne, as his high damage output gives back around 80 health per hit due to the lifesteal. However, once you have completed these you are at the weakest part of the build, survivability wise. If it's a fairly even game, then you're enemies will have decent items too. Without any major amounts of armour or MR, you are again quite squishy. We then come to the situational items.

As stated above, a good player needs to be able to vary his build to suit the game. These items are suitable for using with Nocturne, in certain situations, and can usually be used anywhere in the build. (As in, any order.)
Typically, you won't complete the core build set described above. If you do, then you should start building situationally. More often though, you will need to add more survivability items in your build to stay alive. Of course, this depends on the enemies team composition, but most of these items are defensive.

Defensive

Good against certain character, usually autoattackers such as Master Yi, enemy Nocturnes or Twitch. Needs to be coupled with some more health to be effective.


Good if you keep *just* dieing in fights. It won't make a huge difference, but will keep you from dieing if you teamfights are close. (as in, they win by around 2-300 health, this will allow you to escape or finish them off.)


Great against CC heavy teams. If I see a Lux, Sion or Rammus I will always take one of these. Both are good against mages aswell, but there are better for pure magic resistance.


Nice chunk of magic resist, nice movespeed, nice regen. Great against poke and burst mages in particular, but mages in general.

Offensive

Don't take this as a first time Nocturne, please. Only take this if you KNOW what you're doing, and they have a bad team. Hard to use, but has a huge reward if used well.


MORE SPEED? YES PLEASE!


You'll probably never afford this, but it works reasonably well on Nocturne for some heavy burst damage.


Even more armour penetration can never be bad, but you don't really need it. You don't need the attackspeed either, just get another damage item.


Good against armour stackers, that's about it.


Good against health stackers with no MR. If they have MR, this is pretty much useless. Semi good for jungling though.


More damage, more lifesteal. Pretty self explanatory, but you shouldn't take this if you're gonna die or get CC'd.

No-go items

There are few items that are an absolute no-go. AP, specifically. And these:


WORST ITEM EVER. THIS IS USELESS. YOU WILL GET MASSECRED BY Madred's Bloodrazor. DON'T BUY IT! :D


Waste of money, if you're careful you don't need it.


I don't care if you think this is a carry's GOD. Not good for this build, unless you have a Phantom Dancer, Bloodthirster is a better choice.

First of all, this AGAIN should depend on your enemies. I don't want level 5 scrubs voting this build down because this didn't get them 90000/0/190000, ok?
Take all this with a pinch of salt, and remember that practice makes perfect.

First blood

If you have a team with no-one afk at the start, you should go for firstblood. Earlygame, you have one of the highest damage outputs in the game, second to maybe Master Yi or Tryndamere.
[For those unfamiliar with 3v3 play, first blood is usually in the bottom bushes ;)]
Preferably, you want the strongest champion to check the bushes first, or a stealth character if you have one. You shouldn't do this, but be ready to help them if needed. A Duskbringer/ Umbra Blades combo should scare them off, and if necessary you can use Heal to help them. You can, and should, play aggresively in this phase, as very few champions can stand toe-to-toe with you. If any try to, they're stupid. Generally, I keep Duskbringer for when an enemie tries to escape - this allows easy chasing, and a little bit of burst damage. Exhaust is also really useful here, no escape for them, unless they brought Cleanse, which is unlikely. If you managed to get a kill, you should either get the hell out of there or try to finish off the others, depending on how you're doing.

Laning

Last hitting is hard with Nocturne due to his Umbra Blades. To counter this, an efficient method is to push your lane (preferably top) to around 2/3 in their direction. Once this is done, go do a round of the jungle. This allows you to keep up with your opponents in XP (if not level faster), without pushing you lane too much and get more gold than your opponent. Jungle-wise, you should be able to kill each of the wolf camps in the top lane easily, then return to lane for a bit to heal up using Umbra Blades.
While you are in lane, you should play aggressively. Still, very few champions should be able to match your damage output. However, you're still squishy, so keep that in mind. You'll have no chance against a character such as Xin Zhao, as he'll just use Three Talon Strike on you. Early-game your weakness is CC. If possible, you should harass your opponent as much as possible, if you can harass them out of lane or get a kill, great.
If you are having problems, the best solution is to ask your team to gank them. Nocturne is a natural ganker, and an insane chaser. Combined with a decent teammate, you should have no problems killing them. Even if you don't manage, that doesn't matter. It keeps them away from gold and xp for a little while, which is only good for you.
Whenever you can, GANK. In 3v3 it's so quick to get from top-bottom and vice versa that ganking is amazingly easy, ESPECIALLY with your ultimate. Your ultimate can take you from your top turret to your bottom turret, easily. Keep an eye on your teammates at all times, almost as if you're playing Shen. However, you should remember to be careful even when helping a team-mate. If your mate is nearly dead, and you come and help away from your turret, you get left 3v1. Guess what happens then?

Midgame/Lategame

Gank. Gank. Gank.
Unless your enemies are stupid they will start bunching up together. Best strategy is now for your two teammates to take on the 3 of them. This sound stupid, but it works if your team is tanky - remember this, TANKY. If you're Twitch and Teemo, this won't work ;)
They should focus on 1 person. When said person turns to run, you use Paranoia and Duskbringer to take them out, then help your team-mates. Congratulations, you team just earned 3 kills and some time to push.

• Save your ultimate for finishing off low-health enemies with Flash or other teleport spells.
• Play aggressively, but remember to keep an escape route. Duskbringer allows you to use minions as an effective barrier when escaping or kiting.
• Remember that Duskbringer gives you bonus damage while on the trail.
• When going for a kill, don't forget to use Unspeakable Horror as early as possible in order to chase.
• Most importantly, practice :)
• And remember to have fun :)