Basically Chorizotarian hit the nail on the head.

You basically have to have the good gear before you can do the instances and you can't get the good gear with only 3 toons. If you have the funds, you might be able to get away with it by burning consumables to enhance your toons, but I'm talking a mana pot on most of the trash pulls. Even then your healer is probably gonna go OOM before your lock kills everything.