If I had sex with a guy on the 15th of Dec 2011 and a different guy on the 17th of Dec 2011 and then have sex with a guy on the 19th of Dec 2011 my due date is Sept 10 2012 which guy is most likely the dad

With dates that close there is no way to tell. You will have to have a DNA test done with all 3 men to find out which one is the father.