Unfortunatly I have yet to be able to keep any type of shellie, but they are on the top of my list (right beside the julies).
No the tank will not look full when you first start with just the three, however as I understand it if you get a pair or better, they will be able to saturate the tank before you know it. If you so desire you might be able to get away with some surface dwellers like danios. You could also make the tank look fuller with proper placement of rocks and some plants.
IMO, the joy of keeping shellies, or any other Tang fish for that matter, is not but to watch the number of fish you have, but more the behavior of the fish you have. The idea of a fish whose total world only exists within a few inches around its shell is just so interesting to me. To watch them interact with each other, and take care of each other would just make the tank for me.
Good luck with what you decide!