Losing weight that fast is very unhealthy. The recommended weight loss is one to two pounds per week. Any weight you lose in the first few days of heavy dieting is typically water weight and it does return immediately after the diet has ended. Rapid weightless can lead to some serious medical issues. If your looking to drop about one pound in three days then water pills (sold at the pharmacy) will help strip off some of the water weight but be aware it will come rushing back.