Considering 2/3 Boxing

Hi! I have been running 2 accounts (one for my main, another for alts) for a couple months in order to power myself through instances and to grind honor while simultaneously playing an alt, but I've never thought about genuinely multi-boxing before with the purpose of leveling.

I was considering 3 accounts, with 3 DPS classes (SPriest and 2 locks comes to mind) so that I would be able to potentially find groups once they're ready for instances (and eventually raids).
I guess my question is, is it viable to multi-box in an instance or raid environment?
I have a high-performance gaming desktop and secondary machine capable of running 2 instances of WOW smoothly, so I guess my question is all about group-ability.
I'm not really interested in PVPing them, if I make them.