I had a stall at 3 weeks as well. I was not eating more than 800 calories, and did not lose any weight for 3 weeks. I am now 6 weeks post-op and the scale is moving again. In my opinion, I think it was just my bodies re-acclimating themselves. In the last 2 weeks, I have lost 6 lbs. In my case, I was also very sedentary, not moving and not exercising much.
Everyone is different. I think that if the scale is not moving up, then you are doing something right. Just wait it out. Try not to weigh yourself often. And the scale will start moving again