For PvE, almost any combo is viable. But 2 melee range classes at once, especially rogue + anything is going to be REALLY hard to play.

Don't even think about running 3 WoWs on a single machine, unless you have a quad core triple 8800GTX box with 3+ gigs of memory.

Which I happen to have. I still wouldn't play three copies of WoW on it. Pretty sure my rig can handle it at playable settings but not sure if a dual core or lower could cut it with what I would consider playable in high lag / player density situations.

It would be far cheaper / better to simply use 2 or 3 computers. You are going to be switching back and forth so fast, I think you are going to have a hard time, especially with the classes you are proposing.

Think carefully about what each class has to do - their position requirements, their tanking ability, how you intend to run heals, form shifts, etc. What exactly do you want to achieve in PvE? A fun and interesting group? A powerful group?

Of the three you suggested, priest mage warrior (holy trinity) sounds the easiest to manage and most likely the most effective for most requirements. But YMMV and what you are looking for may not be the same as me.