I think I had a miscarriage, I had a small sac fall out of my uterus today after a week of spotting is that possible with a three week pregnancy?

Miscarriage. Yes that is very possible and likely to have occurred. If you are bleeding more than a typical period be seen immediately.. Otherwise, you should stop bleeding in the next few days. Any concerns see your doctor.
See your doc. It is possible to have a miscarriage but you do need to see your Physician for exam and blood work.