A 21-day water-only fast, so that your body receives none of the necessary, daily vitamins and minerals? For three weeks? Well, you'll probably be hospitalized before the end of it, if that's your goal.
I can't write a nutrition essay here, but a lot of people succeed by eating three small meals a day, from "whole" – not processed -- foods. Eat some lean protein each day, so that your body doesn't literally consume its own muscle, as well as the fat that is probably your focus. Vegetables have fiber and are thus filling; and overall, low in calories. And your body needs all this.
I'm a little stunned that a 21-day water fast would even be considered an option if you have spent time reading about diet and fitness from reputable media. You appear to be having trouble distinguishing between healthy and harmful regimens.
Three weeks of a water-only fast is simply abuse to your body. Do a little self analysis. If you might be prone to problematic eating habits or disorders, CONSULT YOUR DOCTOR OR A THERAPIST about your diet plans before you begin anything so radical!