There are a bunch of different combos, each with their own ups and downs. Really, it's going to come down to what you want to play, but I'd seriously consider the extra effort and lack of cohesion each different class will bring to your setup. Having 2 mages 1 lock might sound great on paper, and might be great in certain situations, but is it worth the extra effort of managing separate keybindings for the class or suffering through things like different spell cast lengths desyncing the slaves? Again, you have to make this call, but most of us have concluded that no, it isn't worth it. This is even more true (IMO) if you plan to pvp.

Given that, I'd recommend that you consider all 3 dps class as the same (that still puts you managing 3 diff classes). Given that, I'd go 3 mages. One thing that helps mitigate the extra mental overhead of multiboxing is that fact that you can blow shit up fast. Mages are, as I'm sure you know, generally better burst on demand than warlocks. Also, if you can give up the notion of the holy trinity (I know it's hard), 1priest 4mage is a compo known to be effective in lots of places in the game (especially world and BG pvp). Running solo instance won't be your forte tho. Nor will arenas.

Lastly, don't bother with the forums. In my other WoW life, I play a mage as my main, about equal parts pvp and pve. I won't claim they're without issues, but the bottom line to me is that they are fun to play, and if you don't suck can be competitive. The forum whining comes from groups of people who saw some mage wind up a ap-pom-pyro combo once and gib somebody and figured that's what playing the mage was all about, so they rolled one. Now they're all bent out of whack because it actually takes some effort to kill somebody, I guess. In any event, the class isn't broken, I expect it will get some (more) help in the future to be more in line with others, and most important (again), it's fun - what else matters?

P.S. I know you want your priest in the mix, but there's a reason so many people run 4-5 shaman. The synergy and simplicity of 5 of them just isn't equaled for m-boxing, IMO.

Anyway, hope that helps some. Good luck.