Ask the doctor for an ultrasound, it might help. Are your cycles regular? If not, the possibility is that the app that says you ovulated on the 30th could be incorrect. If you did ovulate on the 30th, either guy could be the dad, it wouldn't matter if one was three days before the other, sperm lives that long in your reproductive tract. (I assume you are saying the sex was unprotected in both cases.) But if the ultrasound suggests (by the size of the embryo and its developmental markers) that you ovulated later or earlier than the 30th, that might help you rule out one guy. It's a long shot but it is all you have, short of doing a prenatal DNA test.