My period hasnt stopped for 3 weeks ?

How frustrating to not have a parent to talk to about this. He WOULD understand if you simply said "Dad, I need to see a Dr. because my period has lasted 3 weeks. Will you make me an appointment or should I?" There are a number of things that could cause it, all easily fixed, so you need to find out which problem is causing it, so you can get the right remedy. Does your school have a nurse? Maybe she could help. If you have an aunt or adult family member, you could go to them if your Dad won't get you help, but don't go to them first because your Dad will feel very betrayed and unloved if you don't go to him first.