You can! But I would not recommend it. I mean how much weight can you possibly lose in three days. And fasting is never a good option. Your body requires around 1500 calories a day to function properly and if you do eat then you will not be able to do anything. You might end up fainting regularly if you fast for three days in a row. This causes more problems than it solves and you should take care while choosing a fasting plan. Eating healthy and clean food is the best way to lose weight naturally. Fasting is never a good option.