Unless your experienced with fasting I'll say you can expect it to probably be the hardest three days of your life. Bringing all your nutritional and caloric intake to a screeching halt will do wonders for your body but not in a good way and it will not be the least bit healthy for you, I would highly recommend tempering yourself for it, take a week or two to ramp up to it by reducing raw material foods and eating soups only first and do not extend it beyond three days and if you do go pure liquid diet make sure you get at least a gallon fluids in you each day.
A three day water fast reduces your body to 0.00000 nutrional and caloric intake for three full days, I promise you that you will be miserable and three two five days is how long it takes to normally get over the pains of the initial fast. Your going to see the absolute worst of it and that's all you'll see and you'll need to ramp your body back up to solid food so that means soups and stuff for few days after the fast too. Extended periods of time without solid food before suddenly eating say a steak can potentially kill you or make you sick as a dog.
I actually don't recommend you doing it at all and honestly don't think you know what your getting yourself into. Reducing calorie intake damages and can almost permanently damage your metabolism, that's why calorie reduction of great proportion doesn't help you lose weight and can be permanently damaging.
I recommend you do a juice fast first, just juice for three days. It will be extremely hard but you will at least be getting some nutrition and calories into the body and probably make you glad you didn't try just water. Remember that your body still needs fuel so you need to be drinking A LOT of juice probably close to a gallon a day. If you decide the juice fast stick to the natural juices not the mostly processed sugar and not real juices, read the nutrition labels.