Why have I not lost any weight for 3 weeks I'm very active and on a 1500 cal diet?

Mathematics. If you have been on diet for three weeks you should not expect to see much. 3600 cals in 1 lb of fat. Daily caloric requirement is about 1800 cal/day. If on a 1500 cal/day diet you are losing 300 cal/day. Therefore 12 days per 1 lb wt loss. In three weeks maybe 2 lbs. This is less than the accuracy of your scale and normal wt fluctuation. Keep it up. Write in 6 months with 15 lb loss.
Macronutrients. Look at what you are eating. Not all calories are the same. Carbs, fats proteins cause different body responses. Look at your carbs and proteins. Too many carbs and to some degree too any proteins can cause your body to store fat. As well as stall fat loss. Your diet should be low carb, med protein, high fat. Eat real food. That's it! do this and the results will follow.