You should talk to your doctor about this before doing it. You can take risperidone on an empty stomach, but that usually means within a few hours of eating. There may be some risk to taking it while not eating for days that hasn't been studied. Your doctor also knows your health history, and will know whether or not water fasting will be safe for you specifically.
There are some risks to water fasting - https://www.healthline.com/nutrition/water-fasting
So, if you have a bit of a complex medical history in that you are on different medications because you have diagnosis of one type or another, it would be really important to talk to your doctor first before doing something rather drastic like a fasting water diet. That's not probably the healthiest way to lose weight. Three days of that is a hunger strike. :>) Any weight loss from that would not last. There are better ways to handle trying to lose weight and we can discuss that. But definitely talk to your doctor before attempting that.