Let's rule out Guy 2, since he was in February and not March, and you did have a period.
Now, let's see how useful your due date is for trying to figure this out. When and how did you get your due date? Did the doctor do an ultrasound and then based on what he saw on the screen, tell you a due date? If that is how you got it, what week did he say you were in your pregnancy at that time, and what day was the ultrasound? (Like, 7 weeks 1 day on May 24 or whatever). If your due date wasn't given to you by an ultrasound, how did you get it?
Was the sex with both Guy 3 and Guy 1 around March 8 and 12 unprotected? Do the guys know about the situation and are expecting to do a DNA test?
Hi, when you say "Guy 1 was in February the beginning unprotected and guy2 wasn't February at the end on the 17th" I don't think that is what I was after.
I just want to know about the person you had sex with on March 8 and the person you had sex with on March 12.
Got that Guy 1 was on March 12 was unprotected. You said in your first post "I had my period on March 5 I had sex with guy 3 and on the 6&8 also with guy3." I take it that you had sex on March 6 and 8 with Guy 3. Was it also unprotected?
Unfortunately, tenth-week ultrasounds are not pinpont reliable for determining when you ovulated and conceived, they can be +/- 5 days. So let's look at the reliability of your due date based on your ultrasound. Have you had lots of other ultrasounds? Did the due date the ever get changed from information from a later ultrasound?
I think you cannot rule out Guy 3 or Guy 1. You would have even had sperm from both guys alive in your system at the same time, on March 12. It would be nice if your ultrasound were able to say that March 12 is too late, since that would rule out Guy 1, but unfortunately with your first ultrasound being in your tenth week, you just cannot be entirely sure. If I were going to bet, I'd bet on Guy 3, but I wouldn't bet very much money.
Sorry, but you will need to do a paternity test with both Guy 1 and Guy 3 to get a definite answer.
If your doctor has access to an ultrasound done earlier than your tenth week, his guess of March 5 might be useful. But if you didn't have an ultrasound any earlier than your tenth week, the margin for error still applies, meaning you do have to test with both guys. Guy 3 might have an edge but this does not rule out Guy 1. You need the DNA test.