There are several good lineups for 3vs3.
The best is to have a support/dps/realtank lineup.
The champs that work best in this combo are:
1. Janna, amazing support, she shields, slows, heals and knockups, what else do you want?
2. Morgana, decent damage, nice black shield and an AOE stun+dmg.
3. Nunu, great slower, nice ultimate and he can tank and deal pretty good dmg.
1. Xin zhao, this guy is amazing when babysitted by a support + he has everything a good carry needs.
2. Jax, hees really viable in 3vs3, this becuz theres less CC around and he can leap over walls, so great mobility.
3. Sion, maybe it may sound weird, but a good sion is a beast in 3vs3 (go look up KoK Tidus, then you know what im talking about)
4. Akali, when played right, shes a beast. Period.
1. Rammus, when played right, rammus is a beast, he taunts the enemy carries away from you're supports + carries and has decent damage and high mobility.
2. Shen, same for rammus, nice taunt, global teleport to jump in and decent damage.
3. Galio, he can be really annoying with his taunting ulti and has the ability to ''poke'' his enemies.
The way you want to play the game is sending the tank top and let him dominate top, Dps + support down. This way you can pressure up and forcing them to stay at their towers while bottomlane will continue to push. After a pushed creepwave, get the red buff (imo the best buff in 3vs3 becuz of the DoT and the slow) for the carry.
Thats how you play high elo ranked games ^^