I had my period on 8/12/15 i am now pregnant, cycle 30-33 days. i had sex 19-12-15 with a then with b 21-23/12/15 who would most likely be the father?

Tough to tell: Ovulation in a 30 day cycle happens on 30 - 14 = 16, in a 33 day cycle on day 19. If the period started on 8/12, then 1) plus 16 = 8/28, plus 19 = 8/31. since there was no intercourse around these dates, conception must have occurred much later. Without a gestational age determined by ultrasound, either one of them could be the father.