after 3 weeks or after one week is too late to talk about avoid pregnancy, usually avoiding pregnancy shud be done b4 having intercourse.

birth control pill is useful within 3 days after an intercourse, the faster it is, the higher rate for it to success. So it is out in ur case.

As far as i know, avoiding pregnancy is too late for now, can only see if u are pregnant or not.
Unless u wan to try pineapple juice or liang cha but those are not scientific proven to be useful (correct me if im wrong)

Another thing is, just incase, i mean just in case,
u are pregnant, be a responsible person, hope u partner and u will be responsible for wat both of u had done and dun just eliminate the little life,
killing it and thinking that every problem will be solved will be useful for awhile but it going to be a regretful thing until forever as long as ur soul is concious, and bearing it will be painful for awhile but it can be a good thing as long as both person willing to commit together