Well if you're looking at PVE goals, and you want 3 toons. Set yourself up with a solid base of Tank, Healer, DPS.
Personally I would think P-Pally, R-shaman, and Mage. Getting them to work together might be a little harder than 3 of the same class, but for dungeons you'll only have to find some random DPS if your friend isn't around. This way you won't be hung out to dry trying to find a class/spec in LFG channels to run dungeons.
Non-elite world mobs will go down pretty easy with lightning bolts/arcane missles, Or you could round 10 mobs and aoe them, making questing pretty easy aswell.
Just something to think about.



-Xenostar