Hi, L.,
I realize this response comes awfully late! At first I didn't think I had anything worthwhile to say on the matter. But I've been reflecting on my relationship with my mom lately and just came across this email in my inbox.
Your situation is quite different in the details from my parents', but perhaps not in terms of the overall effect on you and a possible fourth child. My mother also wanted a girl, but I don't think she ever really enjoyed me. She definitely seemed too stressed out to bond. In her case, she had two babies in diapers, and then two toddlers to chase around, and one directly followed by another round of terrible twos. My brother and I are only 14 months apart.
I'm answering a different question from what you asked. But as a daughter who has always longed for a bond with her mother, who always wanted that bond, too, but was just unable to get there, I want you to seriously consider whether you will, with three boys - one with Bipolar Disorder, ADHD and Conduct Disorder - and joint custody to juggle - in real life be able to have the mother-daughter relationship you want. Perhaps instead you could get involved in a local girls' organization like Girl Scouts or softball. Truly, you might be able to bond with a girl who doesn't have the mother she really needs, or female role model, either, and you get a great relationship without all of the stress.
I hope this doesn't put a damper on your hopes. I just want you to know that, after living on the inside of a similar situation, I wish it hadn't been that way for my mother or for me.
L.
PS oh, and I have two older brothers. I think it can be great to have that balance, seeing things from a male point of view, but it has made it difficult for me to communicate with women! There are huge differences in approach, I've learned only well into adulthood. But I do really enjoy the balance.