Stay the course- following the diet as directed by your dietician or surgeon, and the scale will move on its own. The three week stall is just your body's way of adjusting. Too early to require changes in diet.
The cool thing is that even though the scale gets stubborn at 3 or 4 weeks, your body is still transforming, for the better. If you take your waist measurements at the beginning and end of the dreaded stall, you will see a change. Your pants are going to be a little looser, despite the same wight.
hang in there! We've all been there and came out great!! (I understand the frustration. My stall at 3 weeks was about a week or so).