I 3 boxed my way to 60 by running instances (Paly, 2x priests - spec'd for shadow, but tossing the occasional HoT (not shadowform obviously)). You may be a level behind on the instances, but it's more fun than just pure grinding. If you can ever find a 4th person to step in then it works well too.

You really don't need 5 people for the lower level instances, but you do need somone that can do some healing. Any of the classes that can fill 2 roles work well for this (Shammy, Druid, Priest). You can keep them mostly spec'd for DPS, just so long as they can still off-heal. With 2 Off healers you have plenty of healing, but it does nerf your DPS a little bit.