I've been three boxing warlocks. Just hit 28 tonight. I have a healer friend and together we can clear instances easily. Most of the time a tank will just slow us down.

I haven't had any experience 5 boxing before, so I certainly can't make comparisons, but I'm having a lot of fun 3 boxing. The only real drawback is No Healer. I'm thinking I maybe should have rolled 2 locks and a priest or something.