I took 3 tablets of postinor 2 in 3 weeks (I had sex once every week). 1)What side effects would this have one me. 2)Would this prevent pregnancy?

Emergency contracept. Emergency contraception should be used within 72 hours of unprotected sex to decrease the risk of pregnancy by about 70%. It is not 100% effective. IT should not be used as your primary birth control. Since you are having to use it often it would be a better option to start birth control. This would give you more protection.