I can't be bothered making this guide pretty.

Bush Ganks
Your saplings do the work for you here. Just toss one into their bush then go in since you now have vision. Or you could stall in your bush if they're all melee and sit there with your saplings.

Jungling
I haven't jungled in 3v3 before. Will report back if a success.

Laning
Poke with your E every once in a while, and if they come close, knock 'em back with your Q. Try to bait their abilities out to proc your passive. Getting a spammy ally really helps in lane, but you could solo.

Teamfights
Easy. Throw up your ult the minute they initiate/commit, watch them scramble out of it, then tap it again right before they get out, and hit their faces. You are most likely the initiator, which isn't a problem - Maokai's tanky enough regardless, and can't be targeted while in Q.

Notes
Sapling are like Teemo's shrooms but they chase you and have a far more consistent uptime. Also, you can't just use an oracle's and destroy them. Sure, they're only up every 35 seconds, but if you manage them properly, you're ungankable.
Maokai's ult is amazing in 3v3.

Maokai's Saplings
Have a total AP ratio of 1.0. His W and Q aren't bad either, and his passive makes him unkillable in a spammy team comp. Enjoy!

Ganking
You are an amazing ganker. Whenever you are even a little overextended, just head back and gank.