Hello, I would be happy to help.
Based on a due date of 12/13:
1. LMP would have been 3/8/2016
2. Conception would have occurred on 3/22/2016
3. If the due date was based on an ultrasound at < 10 wks, then the window of conception would have been between 3/19-3/25.
4. Based on this, the intercourse on 3/20 is more likely to have resulted in this pregnancy
5. BUT, 3/18 is close enough that this can not be certain
6. I do not know what test you exactly did, but the only test that is truly accurate is one that you do directly on the newborn AFTER the delivery has occurred.
In answer to your question - the only thing you can do is wait. You must find out who the father is and you must be honest with these two men. They deserve to know who is the father and who is not. Talk to the OB Provider who is taking care of you and they will help you to sort out how to approach this testing. I hope that this helps!
Please remember to rate this answer as POSITIVE and I will be happy to answer any further questions.