Grapefruits are a proven weight loss food. But having said that, even they need some time to work and three days is too short a period. If you want to lose weight with a grapefruit diet, then you will need at least a couple of months. Grapefruits and other fruits that have high vitamin c content help the body burn fat and you will lose good amounts of weight in no time. Grapefruit or not, nothing short of a miracle is going to help you lose weight in three days. So don't get your hopes up to high. And don't say I didn't warn you.