Had unprotected sex on the 3rd march first day of my period with an ex. Then I did it with my bf on the 15th 16th march. My due date is 13 dec
It is unlikely you would get pregnant when your period is just starting.
Normally you will ovulate sometime between days 11 and 16 of your cycle. Sperm can survive UP TO about 5 days.
Having sex on the 3rd, there would not be an egg to fertilize by the time the sperm died.
The 15th and 16th, days 12 and 13 of your cycle, you would be at/near peak fertility. This is when you likely conceived. Odds are very good your bf is the father.
A DNA test can confirm.
Hope it helps.