I had sex march 12 but got my period march 15 can i be pregnant?

Very very unlikely: Highly unlikely, but your should be more careful and you wouldn't be worried and needing to ask about it. Remember, unprotected sex is the overwhelmingly most common cause of pregnancy.

Not likely: It is possible, but not likely. It takes about 9 days after you conceive for a home pregnancy test to show that you are pregnant. If your bleed was on schedule for a normal menstrual cycle, then it is even less likely. Typically the only time you can get pregnant is about 12-15 days before a menstrual cycle.