I agree with MissymyAngel!!
I agree with the other posts.I know thats not what you want to hear, but you cant be 100% certain. The odds are good that it is guy B's but letting him get his hopes up and then finding out later its not his baby would be devastating to him.
Hopefully you are in a situation where you guys were just "casual" at the time, so you could explain to him that there was another guy you had sex with that month ( you don't need to tell him it was the same day) but the baby is most likely his, and you would like for him to be the "father". Worst case scenario, he wants a paternity test, best case scenario, he decides he wants to be the father no matter what the outcome. My husband is a really great father to my step-daughter, and she is not his blood. He started dating her mother when she was 3 months pregnant and he didnt find out until she was six months pregnant.
Chances are it's Guy's B baby but again to be certain is a DNA test is recommended. Good Luck!
Your odds of it being guy B's baby are as good as the condom was.
I'm sorry, but you won't be able to have certainty until the baby is born and you have a DNA test. Since two of the sexual contacts were on the same day and one was only 5 days later, you're going to have to test to be fair to everyone (especially the baby). There is no way to be totally sure now.