Three different classes is more challenging to play, but I like the Shadow Priest + Druid + Mage idea. The good thing is that you won't be competing for gear with your friends, only your own Mage and Priest will, and you get the whole range of buffs.

On the other hand, more mages means more sheeps. I like to be able to CC three mobs at once with my holy trinity group (Pala, Priest, Magex3).