Ranged aren't viable anymore, you should go with a full melee team and use IWT + stop/backward key to control them.

You don't have a lot of choices about composition, it'll depends on how many characters you run. If you run 3 I wouldn't recommand you to focus on PvP. As long as you are against solo players it'll be easy, but as long as you'll be against more, you'll be dead. You won't have enough dps to kill them faster than they will kill you.

If you really want to do 3 characters I would recommand you to play something like 3 rogues or 3 feral druids, so you can chose when you want to gank and go stealth again.

If you can go with more (10 characters is nice for WPvP), you can go with Unholy DKs, it's the most viable class as far as I know, and you can switch 1 or 2 DKs for healers if you want more survability. You can also go with Enhancement Shamans or Paladins but I don't think they're as good as DKs. As long as you'll kill some people, they'll make large groups to kill you (especially if you have several characters on Bounty).