The game is much more than just arena. Banking on high ratings for arena is not a good idea to start the team. Especially, if you are new to multi-boxing, you will get your hopes destroyed and think that you have wasted all those time leveling and gearing up the team.

For MBing in general, shamans are still king as I see it considering both PVE and PVP (bg + arena + outdoor pvp). Melee team is a new thing. We are all trying it. I feel like they scale better with gear than shamans, meaning they sux big time to start. pwn big time once I get gear. My shamans were ok even with lower gears.

As for playing 3s, since you are limiting to 3 of the same class. I don't think druids will work (get to high ratings). You are not buffing each other. And all 3 have the same problem. LOS. no protection against anything. High armor is all you got.

Outside of arena, druids are fun. Outdoor pvp king as they call it. I still don't know how they can attack me for a while and then instant fly form fly away lol.

For arena specifically, I suggest looking into shamans, warlock, mages for casters team. And paladins, rogues, for melee team.