The exp is slower, but you do get a modest little exp bonus for teaming up if you 3 box. I'm 3-boxing with a Priest and two Mages. What I find is I have to swap between two areas to gain a constant flow of exp. Some of the leveling guides out there even suggest good quests to pick up in an area to help make things work smoothly. For the most part though, it is a tad pain in the ass, but eventually you'll feel the coolness kick in and it's a lot of fun. Right no I'm in my mid 20's and I'm starting to face some mild issues like dealing with Elites, managing aggro better and messing around a lot with Macros. It takes a lot of work, but I'm getting better at it all the time. One thing you have to realize, is that over time, you need to learn how to split your attention. It takes some getting used to. I'm still bending my brain around it and it's not that easy to do, honestly.