I had sex 3 days before ovulation and i got sperm inside my vagina why didnt i get pregnant??

Few couples conceive after only one or two tries. Often it takes a few months. If you go a year with regular mid cycle sex without becoming pregnant, that's when to consider seeing a gynecologist to evaluate for possible fertility problems.

Many things other than sperms in the vagina are involved in getting pregnant and it could be any or all those reasons