Henmommy, The three in the equation comes from the idea that each pound of bird needs three cubic feet of space, even if much of it is above four feet. According to my calculations from what you said, you have room for all of your birds if they are about ten pounds each. I think many breeds are...
Unless you have banties, you've about hit your limit. You might get away with one more...or you could always do what I'm doing and build a second coop. lol