Throw out your scale and get a tape measure. If you’re losing inches, then you’re on the right track, no matter what’s happening to your weight.
On the other hand, the most you could safely expect to lose after three weeks would be about 3-6 pounds, which may well be within the margin of error of your scale.
For the next week, keep a journal of all the food you’re eating. See if you can’t get a rough idea of the caloric content, and record that too. You may be eating more than you think, or you may not be eating enough.
What’s your age, gender, and height?