Mundo is a really underplayed and underestimated champion. His power comes from his heals and the power to destroy tanky DPS champs - the most powerful type on 3v3 - with a few cleavers.

Pros:
-Sustain
-Tons of hp and damage
-Spammable slow
-Counters tanks and tanky DPS champions
Cons:
-Kinda countered by healing reduction such as Ignite or Executioner's Calling

My runes are all flat, aimed at boosting your early game performance.
Magic penetration reds help your early game cleaver damage a lot.
Armor yellows have got to be my favourite yellows - they nearly double your starting armor. I'd recommend them on almost every champion
Magic resist blues boost your tankiness.
And health quints in combination with the Veteran's Scars mastery give you about 100 health more on level 1. And that is a ****load of health for a level 1 Mundo.

Starting with a Doran's Shield gives you three important stats on Mundo: Health, armor and health regeneration. Another possible start is Boots + 3 potions but I find that you are a bit squishy that way.
Your next item should be basic boots (if you don't start with them). Sometimes it's good to finish a creep wave, go kill a small neutral camp without having to worry about your hp much, and then go back, heal, and buy your item.
Next you should build Warmog's Armor. This is a staple item for Mundo and it's very powerful early.

Boots choices

After your Warmog you should finish your boots. The best boots for him on 3v3 are Sorcerer's Shoes because of the magic penetration they give you early. With it your cleavers - the main source of damage in early game - will deal massive damage.
You might even choose to finish these boots before your Warmog.
Other boots choices are Mercs against a really CC-heavy team, which is uncommon on 3v3. I'm a big fan of these boots, even on TT, but with the cc-reduction you already get from your W you won't be needing them.
The other boots you might want to get are Ninja Tabi. They help against autoattack-based champs such as Tryndamere or Fiora, but you will have armor from other sources and so you probably won't need them.
After your boots, pick up a Spirit Visage. This item looks like it was designed specifically for Mundo. It's cheap and all stats it gives are good, especially the healing and regeneration increase and cooldown reduction. Also the magic resistance aims to balance your armor and magic resist so you won't get destroyed by one type of champion.
The choice of the next item could use a bit of explaining. When you buy it, you should be in mid-game. People will be around level 10 and more fights will start to happen. This item boosts your damage output significantly because of the combination of cca. 150 AD you will have with your Masochism and your fast attack speed. At this point of the game, your cleavers will stop being so devastating and you will have to deal some of your damage through Burning Agony and your autoattacks. The movespeed boost it gives is really powerful, too.
Your fifth item should be Atma's Impaler. With around 3000 max hp this item will give you 60 damage. That's like an unstacked Bloodthirster. The other stats are useful, but not as important as the damage.

Sixth item choices

The Bloodthirster - If you're doing well and you don't feel some character in specific is very dangerous, go for this item. Increases damage nicely and gives you sustain without having to use your ultimate, which you should save for teamfights.
Guardian Angel is a more defensive choice not aimed towards one type of damage.
Thornmail - If you're having Tryndamere or Fiora problems, take it. But beware, if they have Last Whisper, this item won't be as effective - 40 of your 100 armor will vanish.
Force of Nature is a powerful item against AP champions and also Madred's Bloodrazor. The other stats it gives are very useful as well.

Level 1 fights in the jungle are even more important on 3v3 than on 5v5.
This is a map of places to stay for first blood. Your base is on the left.
1 - This is the usual place people will go in solo queue. Don't go there. Why? Because:
a) People will expect you there and just get you out with skillshots
b) If they don't have any skillshots at level 1, the fight will be risky and may even go awry.

2 - A good place, often gets you a first blood because somebody uncarefully walks in. You may still get skillshotted out by smarter opponents. If you don't and nobody walks in, you may go for a gank to kill the 2 people at bot.

3 - A defensive variation of number 2.

4 - This spot is aimed at killing the person who goes from place 1 (where they waited for first blood) to the top lane. A lot of people choose to go the smarter way, through their base or close to it. On the other hand, if they don't, you get free first blood. Organized teams may also go together. In that case, a dangerous teamfight will erupt.
If nobody comes, your team can choose to clear the creep camp slightly below your spot.

Spell selection

Generally, the best spells to take first are (most important things first):
a) Crowd Control spells - such as Sion's stun, Olaf's axe etc. These effects are a powerful tool when chasing and fighting.
b) Skillshots - Ezreal's Mystic shot, Nidalee's spear... They are good for checking bushes and dealing damage from afar.
c) High damage abilities - if your team already has 1 or 2 crowd control effects, some champions may want to get an ability that even though it doesn't slow or silence deals big damage.
For example, it is typically better for Lee Sin to get his Sonic Wave (which is also a skillshot) over his Tempest.
As Mundo, the best spell for first blood is your cleaver. I put this chapter in for some general 3v3 advice.

Maxing your cleavers first is a must. The early damage output is huge and you also hit the Lizard Elder and Dragon for 700(!) damage at level 5. This makes getting the red buff really easy.
After my cleavers I choose to alternate between the two remaining skills because I think they're both about equally good, but if you want, you can max one of them first.

You should have one movement spell and one other spell.
Your movement spell choices are:
Flash: Even after the nerf, this spell is insanely good because of its versatility. Flash over a wall to escape, flash to hit that last cleaver, flash to steal dragon, flash to backdoor their nexus when their tower is down... and the list goes on.
Ghost: Some people prefer Ghost over Flash for escaping and chasing. It's nearly as strong on a tanky champion (who Mundo is), but I still think Flash is better because of the number of things you can do with it. But if you're not good with Flash, go ahead and get Ghost. But my recommendation is for you to learn to use Flash.

Spells you can pick to fill the second slot:
Heal: My personal favourite. I believe that every 3v3 team should have exactly 1 heal (having more is a waste due to the debuff). It's great for first blood and early game 1v1 fights. Use it to surprise your oppoenent when you're fighting at low health. Just be careful if they have ignite - the effect is diminished a lot by it.
Ignite: Mundo's nemesis can also be his friend. Ignite is a nice finisher of low champs and can also be used for its healing reduction.
Exhaust: Good against champions like AD Sion or Tryndamere.

Mundo is useful on both lanes but he really shines more at top lane. You can go 1v1 with most other champions and win. That is why he is a great aggresive laner. If you think you can win 1v1, hit your enemy in the face with a cleaver, turn on W and E and go for him. Hit him a few times and see what happens. If he loses more life than you do, you can begin playing aggresively and zone him.
This way you can beat Tryndamere, one of the most feared champions on lower levels, at laning. Remember, Tryndamere is good only if he has a lot of items. Denying him farm and kills makes him useless for about 30-40 minutes. Then he starts catching up and can be dangerous again.
If you start a 1v1 fight to the death with your lane opponent and it's going more in his favour, you can just use Heal and then finish him off, so don't be afraid.
If he is too strong (has good sustain or outdamages you 1v1) then you should play passively, just stay back and lasthit minions. Don't push it or you might even die.

Thanks for reading the guide. Please rate and give me feedback.
Now go out there and kill some Tryndameres.

Arcaanis