3 weeks after sex i took pregnancy test negative result. i also took ecp after 3 hours after sex. is it reliable and accurate? still no menstration.

Yes: It sounds as if you would have had a positive pregnancy test by now, if you were pregnant. 3 weeks is sufficient time for the test to turn positive. 3 hours after intercourse is a great window to take emergency contraception, you likely had close to a 80% chance of stopping ovulation. I think you can just wait, try not to worry. Repeat test in 1 week, but i think you aren't going to be pregnant.