As the opinion of a non-doctor, I would say it's 100% impossible for the other guy to be the father. While sperm get to the tubes within minutes to hours, it takes 6-9 days for implantation to occur after fertilization. I can't even imagine you'd have ovulated 3 days before your period. If you did, the egg would never have made it to the lining in time to implant and would have been flushed out during the 5 day-long period.