- Important spawn timers:
Jungle Creeps [1:40] / respawn after 50 sec
Ghost Relic (Heal) [1:55] / respawns after 90 seconds
Altars unlocking [3:00] / unlock after 90 seconds
Vilemaw [10:00] / respawns after 5 minutes

- As long as you can get a good early game farm (that means 4 camps before getting the first altar) you will get equal farm with the laners if they don't take your jungle

- In case that your bot lane is 1v2, let your bot laner farm at his tower and help him when your tower loses HP

- Snowball as hard as possible, if you don't play ranked, playing assassins is the most effective way to win 3v3s

- Get the first Ghost Relic as soon as it appears

- Farm the enemy jungle when the enemy team has no jungler

- Capture the enemy altar, use altars for baiting teamfights

- Sanguine Blade is an item made for AD carries not casters. Without its stacks it has only 65 AD which is not worth 2850 gold since the lifesteal isn't really useful either. Ravenous Hydra is better because of its Active and it gives 10 more AD.

- Infinity Edge, Ravenous Hydra and Muramana give the biggest AD bonus, get your Manamune early for AD.

- Iceborn Gauntlet reduces physical damage taken by around 30% or more, get it first vs. strong AD teams, especially against Tryndamere.

- Mercurial Scimitar gives a big AD bonus and movement speed (+Cleanse), in combination with Mercury's Treads and Maw of Malmortius you are tanky vs. strong AP teams.

- Ionian Boots of Lucidity are good to increase advantages and to snowball.

- Grez's Spectral Lantern is worse than the Spirit of the Elder Lizard IMO. Someone should get a vision item when it's needed, but not Kha'Zix, he needs the damage and CDR from the Spirit of the Elder Lizard, which is still a good item in the late game.

- Choose your defensive items wisely, for example a Frozen Heart slows Tryndamere's attack speed. You will not be able to get more than 2 defensive items, you will lack damage.

Building Kha'Zix tanky is neccessary when teamfights are long. Kha'Zix isn't made for long teamfights, his job is to burst carries down and run away. Melee AD carries are very controversial, they need survivability and steroids to perform well. Good examples are Tryndamere and Aatrox. Kha'Zix is not one of them.

On 5v5 you would build Bloodthirster in the mid game which costs 3200 gold, but it is not available in 3v3. Manamune costs 2100 gold and The Brutalizer costs 1337 gold, which makes 3437 gold together which is only 237 gold more than Bloodthirster.
Especially the 10 flat armor penetration will increase your damage dramatically vs. carries and bruisers that don't build armor early. Also you will receive ~75 AD, mana and cooldown reduction from only those 2 items.

- Ghost/ Flash is the best combo from my experience if your team has an Exhaust, movementspeed is extremely good on this map (slows too)

- Barrier works fine for quick engages

- Ignite is good against lifestealers like Aatrox and of course good to get an advantage early. Your ganks will be more powerful.

- Cleanse is very viable. It's problematic when you can't use Void Assault or Leap to disengage because you're silenced or stunned and instantly die.

- Exhaust is OK. But to be honest a tank should take this summoner spell. One advantage of Exhaust are powerful ganks because of the slow.

- Smite
Pros:
- Faster jungle clear time -> Earlier ganks
- Counterjungling
- Secure/steal Vilemaw
- More gold in general
-> Faster Tear of the Goddess
- (Smite Zac)
Cons:
- You lack an Ignite in your team -> Problems with Dr. Mundo, Lifestealers, Enemy health regen
- You lack an Exhaust in your team -> Problems in fast teamfights
- You lack mobility -> You will die more often and you will not be able to chase
- You lack Cleanse -> Instantly dead after engage

Armor Penetration
+ +
explanation: Nasus gives flat armor penetration with Spirit Fire, this helps Kha'Zix out bursting carries. Since this comp is not missing a tank or an AD, you need an AP carry/bruiser with CC like Kennen, Rumble, Annie, Cassiopeia, Galio, Singed, Zyra, Malphite or Cho'Gath for example.

Single target destruction
+ +
explanation: When the enemy team has only 1 carry and 2 bruisers/tanks, it's important to destroy that one carry first in a teamfight.

Crowd control combo
+ +
explanation: While Kha'Zix has burst and Fiddlesticks has damage over time all you need is CC, Cho'Gath provides one of the best forms of crowd control on a small map such as the Twisted Treeline. Other advantages of this compilation are that 1. Cho'Gath can handle a 1v2 lane, 2. Fiddlesticks is pretty much ungankable in top since he has a 1-3 second Fear plus Silence.
The CC that Cho'Gath and Fiddlesticks have together is huge.
You can replace them with all of the CC champions listed in the armor penetration compilation.

Evolve Leap first to be able to escape and jump far for ganks.
Evolve Taste Their Fear secondly for the huge burst and because of its low cooldown.
Evolve Void Assault thirdly for survivability in teamfights.
You don't need to evolve Void Spike anymore, it's become almost useless, the AoE effect doesn't do much and you already have the slow.

In all these games I've been jungling.