For 3-boxing straight warlocks, I'd definitely NOT go destruction. They're already mana hogs until 35ish anyway, even specced affliction. My 4 warlocks at 70 are demo/aff and are a very well-rounded group. Felguards chomp face very nicely against anyone with less than 300 resilience.

http://www.wowarmory.com/character-s...n&n=Donbartolo

I leveled with a priest so I had plenty of lifetap+PoH going on but I think I would go with the standard drain-tank strategy since you don't have a healer. The upside is that you'll be able to quest like mad with almost zero downtime with three locks, especially later when you get Curse of Elements and Curse of Shadow going together. The downside is that affliction sucks for doing your own instance runs unless you seriously outgear/outlevel the instance.