Oh, wow! I was thinking just like you before and when I had my second healthy boy. Although I still would like to have a girl, I have decided to put my strength, energy, and money into my two wonderful boys since that is only fair to them. Wanting a girl so bad can backfire if you would get a third boy, and it really wouldn't be fair to that baby since he may feel like he is not wanted. The other boys really need you there for them since having one more child does spread your time and money thin. If you have trouble making time for your two boys now, think about how hard it would be with one more baby to change, feed, dress, etc.
Having two kids is so much easier for traveling, eating out, going shopping, and saving for their education. Remember, you have two hands to hold two children! :) Life can get so crazy, and having a third child would mean needing a bigger car and house probably, and then having less money to spend on fun family trips or other items. It might also mean that you or your husband would need to work more hours which would mean spending even less time with your family.
Of course, some people have three or more children with no problems, but maybe they have a lot of money or they don't mind doing without certain things. I know families with three kids who don't go on trips at all (even the zoo) since it is just too expensive! It all depends on what is best for your family.
You could always volunteer at your boys' school to get your "girl fix" by helping little girls and boys learn (I am a teacher and that helps me). Or you could babysit other people's children for a short time during the day so that they could do errands, etc.
I have learned that in life I have often wanted certain things so badly, but have not gotten them, and it has just made me more thankful for what I already have. I have learned that things happen for a reason that is beyond what I can see. If you push too hard sometimes, things can go wrong. I can totally understand where you are coming from and how you are feeling. Whatever decision you make, I wish you the best of luck!