3 day water fast a month sounds incredible. No problem at all. Just research about electrolytes.
Did you know during Ramadan Muslims fast for a month, eating only the few hours before sunrise? Did you know people in Israel fast for a whole day, without water or food, every year?
I fast for health not weightloss. I usually eat low carb but I really concentrate on eating plenty of healthy fats a couple of days before the fast so my body has something good to burn through while all the other beneficial processes are taking place. I feel very healthy fasting every month!!