Would a pregnancy test show positive 72 hours after conception/intercourse?

No and Yes: No, a pregnancy test won't show a real positive 72 hours after conception, because there is no bHCG from the embryo in the mother's blood or urine yet. Yes, a pregnancy test can show a real positive 72 hours after intercourse, if the woman had already conceived >2 weeks earlier, and just happens to be having more intercourse (as is normally done in life). A false positive is rare, but can happen.