Will water fasting for three weeks provide any kind of drastic weightloss?

Probably: If you mean fasting by drinking water only for 3 weeks, yes you would probably lose weight. This weight may well include muscle, though -- a result that in long term would be unhealthy for you and lead to eventual weight gain. With less muscle than you started with, you'd have less active metabolic furnace. So when you start eating again, more consumed calories will be stored as fat.