well with a 3's group you can practically do any combo you want.

3 mages are fun for pvp - free food - port to anywhere for fighting

1 priest 2 locks are usable - priest good for rezzing and keeping regens on locks while they mana tap- along with fort buff

3 shammys are always a given, self rez and rez others helps alot for when you are tired of corpse runs.

3 hunters are fun - a little harder to pvp against higher lvls since you need to kite. Hard to keep facing the target on all 3 when shooting

any 3 melee class i would steer clear of.... alot of micro management there.

just try them all out and decide