Personally, I’ve done a couple of 48 hour fasts after doing 18/6 for a couple weeks with no issues at all (taking proper electrolytes).
I don’t think it’s possible not to lose any weight doing a 3 day fast. Eat extra on the days leading up to it I guess.
You won’t lose much - and like someone else said you’ll gain it back probably.
I just did a 72hrs fast - broke it on Wednesday night. 2nd day was rough but otherwise good.
Probably going to try a 5-day soon, maybe in Spring.