I have had a period for 3 weeks is there some kind of medication they can give me so that it will stop?

Diagnosis first: Many conditions cause prolonged or abnormal uterine bleeding. Your first goal is to see a specialist and get a diagnosis then you can get the treatment that will help stop the bleeding. You could have a hormone problem, pregnancy problem, abnormal growth such as a polyp or fibroid, blood clotting disorder, etc. Until we know why you are having abnormal bleeding, treatments will fails. Best wishes.