There are classes that have a lot of "save me" cooldowns, once they blow those they become much easier to kill. So for example if you were fighting a RMP (rogue mage priest) all 3 have some great cooldowns, like cloak/vanish/blind, iceblock/nova/blink, pain-supression. So you may want to open up on the mage to force heals/PS/iceblock, quickly switch to the rogue when mage blocks, then try and switch quickly to the priest and get a kill in when the rogue vanishes.

Remember that most teams you face are gonna be on vent so they are gonna be yelling at each other, the more you switch targets around the more confusing it is going to be for them.

I'm running 3 DKs on the arena test realm, I am absolutely horrible in PVP/arenas but yesterday I won 4 games (out of 10) using the "1 button DK macro". Its a ridiculous macro that combines all the DK skills into a single /castrandom button, funny as hell. I'm just playing 200 games to get the murloc pet, but DKs are pretty OP in arena. Sadly they are getting nerfed quite a bit in 3.1

Oh and in my games I'd just start attacking whoever happened to come out first (usually hunter or warrior), then quickly switch to the healer after a few seconds and burn them down a bit, then switch to whoever seems the weakest (druid, hunter, shaman, mage) and try to get a kill in. Obviously only worked 4 times but I had fun