There is a reason people don't overland quest in groups of 5... it just isn't that fun. I toyed with 5 hunters, and had an experience similar to yours. I found that I had the most fun fighting in zones that were 6 or so levels above me. The monsters hit harder, and went down slower, but offered much more XP. Give that a try.

You can also try to solo instances when you outlevel them by a few levels. I'm not sure what the XP will be like, but it should be more challenging.

There isn't much content in the game tailor made for 3 person groups, and Blizz took out most of the overland elites :-( .