I doubt you'll ever be able to clear a dungeon while it still gives xp with only 3. They are meant to be challenging for full groups. I'm running 5 now and have mainly leveled from 18 to 40 so far through dungeons. I've tried many specs and combos and while I've found a few that work well, I have trouble until at the upper range of level and gear for the dungeon. Some are harder than others for boxing but even so, there's really a set point of dps and hps you need to beat most of the bosses.