Well the trick to getting high DPS on a lock at affliction levels (which is until you've got T5 on farm) is to dot up anything with a tank on it. I fail to see how having your main click targetting and pressing hot keys is going to be any less efficient on three locks compared to one. Casting spells at the same time on the same target is the one thing that's easy to do as a boxer. The same goes for demonology and destruction. Infact destruction would be easy mode++ cus all you do is spam shadow bolt.

Of course if you were controlling a toon of a different class at the same time, you'd struggle to meet 100% DPS on the locks unless you were incredibly nimble in the brain and fingers department but all locks would be trivial.