I have my two wondering little boys who is 4 years old and 8 months old... My husband and I always said that we are only going to have two and we love it if we have one boy and one girl... When I found out my second baby is a boy, we were so disappointed, so did everyone else around us which it made it worst since you are already extremely disappointed yourself that you aren't having a girl... I cried for a whole week right after we found out the news because the expectation to having a girl to friends and families are so high. I felt I failed them to have another boy instead of a girl... But as wonderful they are, even though they are disappointed, but they show me that didn't matter if I was having a boy or girl, they're very excited about him and would love him either way...
First, I would suggest that please... please... don't "EVER" mention or show that you are disappointed or still hoping they're having a girl... It's a very bad thing to the mother, especially the mother... It would effect her mood and everything about her... Second, I would show her that how excited you guys are that she is having another boy... show her how wondering her two older boys are... Third, reassured her that it doesn't really matter now she is having a boy, you guys will love him and her very very much.
Even though she will always wondering what her life is going to be if she has a little girl which it will never stop, but this is a very important time to show her that her life is going to be a whole when her little guy is here with her and everything is going to ok.