The human body is good at preserving muscle mass during a fast. It is unlikely that you will lose any muscle mass during a 3-day home water fast. However, it's more likely that you'll lose vital body mass during a longer fast.
To avoid this, most clinicians do not recommend doing an extended water fast at home. If you want to do the longer fasts, you should do so at a clinic where they can monitor you for signs of distress.