League of Legends (LoL) Question: Best 3v3 Character ?

If your team has a jungler:

The strongest champions to play top are mainly bruisers/duelists:
Garen, Darius, Jax, Volibear, Poppy, and Kayle.

While if you're looking to play a waveclear bot laner, you should be looking mainly at mages with a few exceptions:
Brand, Anivia, Orianna, Vel'Koz, and Kayle

If you want to jungle:
Volibear, Poppy, Kayn, Skarner, and Trundle are probably the best.

If your team is going support-smite carry:

As usual, you want a waveclear bot with Brand Anivia Orianna or Vel'Koz

For the support, good options are Taric, Lulu, Janna, and Leona.
For the smite carry, you want to look at champions that can hard carry when they're ahead in items:
Kayle, Xayah, Yasuo, and Jax to name a few.