In just 3 days of drinking water you won't lose much weight and you can only lose about 10 lbs of weight per week safely and anymore weight loss than that is unhealthy.
So you might loose half a pound 1 and half lbs in a week drinking water for 3 days but you need to also watch your calorie intake and exercise during those 3 days and beyond to lose any amount of weight.
Just drinking water won't help you lose much weight but it certainly can help and the water will also benefit your health and lubricate your joints and hydrate you as well.