I just stepped up to triple boxing recently (was waiting on 3.2 for cheap mounts and early flying lessons). I initially two-boxed a prot pally and holy priest up together, and they did great. Now I added in a demo lock.

I have to say, it's a HUGE learning curve to move from 2-boxing up to 3-boxing. It took alot of careful planning, major adjustments to macros (I didn't want to have separate macros to heal each toon like I did when 2-boxing).

My biggest impression of the difference between 2 and 3 boxing is...holy crap do collection quests ever suck! Somehow I didn't really notice it when 2-boxing, but when you have to collect 3x the stuff...now I see why most boxers skip collection quests that don't offer a great reward or open up a ton of other quests. I've already decided that even though the zeppelin debris quest in hellfire offers great prot pants as a reward, I'm not doing it...I do NOT want to have to collect 90x debris!

But I have to say, being able to fly as soon as you reach outland REALLY speeds up questing! It's so easy to "get in, get out" to get a quest done now, you don't have to fight your way through mobs or chance it and ride mounted through hoping you don't get knocked off your horse and have a huge aggro train on you.