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Hello from JustAnswer.
It is common for an ultrasound to be a few days off, so the fact that it is consistent with a last period of May 15th is not surprising. It sounds like you are certain that your last period was on May 18th, and when the mother is certain, many doctors would not alter the due day if the ultrasound is within a few days.
In reality, though. this difference would not alter which guy is the father. The day of ovulation by your recollection of the day of your last period is June 2nd, while the day of ovulation according to the ultrasound would be May 29th. But for either date, if the only day of sexual contact with guy A was May 17th or 18th, then he would not be the father. Sperm can survive in the female genital tract up to about 5 days, but they would not have survived the 11 or 12 days until the earliest of the possible days of ovulation.
So, if your recollection of the dates of sex are correct, the only guy that could be the father would be guy B.
If I can provide any clarification, please let me know.
Please let me know if I can provide any further assistance.