9x3 = 27 square feet. I am thinking 2, maybe 3 birds. You could probably do 3 birds if you add some stuff to the run, some roosts, some hideouts, a pallet on cement blocks, give shade under, and a perch on top.
Whoops, I misread this, I was thinking of a run, do you have a run attached? Or are they going to free range?
Generally, I have found for a coup that they are just sleeping in and laying, not spending the day in, 2-3 sq feet per bird. So about 9 birds, depending on the size of your full grown birds, their personalities. Keep in mind, that many to most problems in a flock come from over crowding.
A couple of points - chicks take up very little space, and grow fast..... so what seems like a large space, rapidly gets smaller. If you free range, and it is summer, you can cheat on the numbers, because it is daylight much longer, they stay outside much longer, and can spread out and get away. However, in the winter you must reduce the flock to a manageable size that fits. The daylight hours drop drastically, and the birds spend 14-16 hours in the coop. You need enough birds to generate enough heat to keep them warm, but not so many that they pick at each other.
Watch your flock, if they are calm and relaxed, you have it right, if they are irritable, crabby and stressed, you have too many.
Mrs K