I may be a bit biased but three classes is going to be hard to coordinate. Not impossible but well, hard to play well.

I would suggest going with priest mage mage or priest warlock warlock or priest mage warlock if you really want to play three.

It also depends of course on what you want to accomplish. Do you want to PvE? PvP? Both? Will you be using one computer or three? The answers to these questions will dictate how successful you are likely to be with each class choice.

Although truth be told, any of the above three combos would be nice, as would be 3 shammys.