My Doctors Have Given Me Your Different Due Dates Starting

My doctors have given me your different due dates starting with November 17 second date was october 17th the third date October 7 now they said November 29th only because my baby is measuring four pounds and small my other two children were only six pounds and four pounds 11 ounces and they have only documented one due date but gave me four all together and the doctors I see I have to see all three instead of just one and I'm not for sure when I'm actually due I'm measuring 36 in the belly and I have only have had three appointments because I didn't know I was pregnant until I went to go get my annual chck up for birth control and my first ultra sound stated I was 29 weeks and that was seven weeks ago what do I do