As previous comment..
But also, my 6 week scan was wrong by a week in my opinion and was told that's impossible, I was sure as i knew exactly when my ovulation was... At my 12 week scan it turned out the earlier scan HAD been wrong by a week.
However, for it to be wrong by 3 weeks at that stage I would say is impossible. 3 weeks is a long time and the baby at 11 weeks compared to 14 weeks is a LOT smaller.
But yes, you'd need a DNA test to be 100% sure.