3 weeks of fasting? This is dangerous. Your body will already be in a state of ketosis at 3 days. It will start to break down protein over extended fasting periods causing wasting of your muscles, your immune system will be very weakened, your metabolism will slow and you will feel extremely fatigued since your body will be trying to keep energy expenditure to the minimum. Any weight you lose you will gain back as soon as you begin eating normally since your body recognizes when it is in starvation mode and adjusts your metabolism accordingly. If you're satiating hunger pangs by drinking water you can cause an electrolyte imbalance, there will be mineral deficiencies, and it will cause hypotension. There's so much more, but I can say an extended fast like this isn't something I would recommend to anyone. And it's from personal experience as well, involuntary though it may have been.
