My gf and i had an unprotected sex for 3 consecutive weeks, she told me to ejaculate inside her. we're only teenagers and can't afford to have a baby. is it still possible to take a birth control pills?

May be too late: Bc pills work only when taken before sexual activity. Your 3 wks worth of sex may have already started a pregnancy. If you don't want to go through that now, take steps to prevent it. Condoms work in the majority of cases & your girlfriend can get on the pill if she is not pregnant already, perhaps both methods would be a good idea, she may want a pregnancy even if you don't.