3x Mage is an extremely powerful DPS core for any comp. You learn arcane explosion at 14, and you will carve through 1-60 with that spell alone. You will get your team to late game content the fastest by using mages as DPS. Other classes will be comparable or stronger as dps post 60, but you won't level through classic nearly as fast.

Tank + healer is personal preference. Paladin tank + priest healer add a lot of early game AOE and are capable of handling any 5man content in the game.