It's almost impossible to lose 15 pounds in three days--and if you do, it won't be permanent fat loss; it will be water weight. However, if you are retaining a significant amount of water and are willing to be strict with your diet, you can lose a lot of weight quickly--though it might not be as much as 15 pounds. If you do not stick to your diet, you can regain all of the water weight.
Eliminate salty foods. If your diet is high in salt, you are more likely to retain water. Cutting back on salt will help you release water weight. Do not add any table salt to the food you eat, and check food labels of processed foods for sodium content. If it's high in salt, avoid this food for a few days to lose the weight.
Stay hydrated by keeping your water bottle at your side. When your body is not getting the water it needs, it responds by retaining the water it already has. Drink as much water as you can, refilling your bottle throughout the day.
Limit or eliminate starchy foods and refined carbohydrates. Sugary or starchy foods can cause you to retain water. Choose healthy foods, such as fresh vegetables or fruits.
Consider diuretics. They help purge your body of excess water. You can purchase an over-the-counter diuretic or look for natural versions. For example, coffee, green tea, lettuce and asparagus are natural diuretics.