Oh wow, keep reiterating the word "free" and tell him that kind of offer doesn't come along often! When you already have two, one more isn't that much different! Show him pictures if you have them, and keep saying how gorgeous she is, and how much it would mean to you to have her. Ask him to just give it the weekend to try it out and see. Then, he will hopefully fall in love with her, and you'll have your girl!
But, whatever you do, don't say "this will be the last one, I promise" because when you want a 4th, he'll just throw that up to you.
Good luck!!!!!
ETA: Oh, and if you get him to say yes, let me know how you did it. I have two boys, and I want a girl now!