I boxed warrior/mage/priest for a long time. Lots of micro-management with 3 different classes, but still fun. Got them all to 70 but my steady office group has kinda fallen apart so I don't play them anymore.

I'm now trying out 3 locks, up to level 26 so far. Locks are already good at taking on multiple mobs, even better with 3. I don't PvP much so I can't speak to that side.

Basically 3 different classes can be difficult to control while 3 of the same class can almost be played as one character but at triple strength.