She has expressed the wish to be a boy since age three and insists she's going to be a boy when she grows up. She speaks of this at least once a week. She has an older brother and sister. I know she is probably just a tomboy, but please advise.
With many children, the enjoyment in wanting their own way comes from the reaction of the adults around them. Think about your responses to your daughter. Do you try to argue with her and give her more attention because of the things she says? What happens if you ignore her or just say, "We'll see?"
Since this has been going on for almost three years and she talks about it at least once a week, you may want to talk with your school counselor about giving your daughter some individual time. If you or the school counselor think she needs therapy, the counselor or your pediatrician can help refer you to someone in your community.