Should I get individual nest boxes, or several joined together?

We have 13 hens, soon to add 8 more, for 21 total. At the moment we have a 3-hole nest box, but they all seem to use only 1 of them. We are adding nest boxes soon, and I wonder if we'd be better off with a few SINGLE hole, so we can space them around the coop... Do you guys think they'd be more likely to use different ones if they weren't all right next to each other in the same spot? Maybe different walls, different heights? And does anyone have a "recommended" height for nest boxes? Currently, ours are about 4 1/2 feet off the ground, on a shelf where the goats can't get to them. : )