I think 4x4 should be big enough for 3 birds. 2 ft is recommended per bird and that gives you about 5ft per bird. Especially if they will have access to outside then they will just be in there at night. I've heard insulation is unnecessary because the birds heat themselves and each other up. My disclaimer is that I am not speaking from experience just what I've researched. Our first flock is still in the laundry room We are still building our coop but here are some pictures if that helps. Used a dog house we got off Craigslist and raised it up on stilts. But we also added on because we have 11 chickies we want to fit. We're working on the run that will come off the front where the "porch" is.