Twisted Treeline

Anyone that snowballs hard and doesn't have to build a lot of damage to keep killing things. For example, both bruisers and melee carries are very strong on 3v3. Ideally, you want to go for champions that are strong early game and can force fights, as most games are decided before 10 minutes. Same pretty much goes for AP champions. You want someone with a strong laning phase and can catch people out easily, but are hard to kill at the same time (i.e. Brand, Annie, Swain, Lux).

If you're good at farming, it's okay to pick a champion with a weak early game as long as they scale very hard into late game. Requires a lot more finesse and awareness than above however.