I don't think that a sudden 3 days fast will bring any immediate benefits (perhaps some weight loss), on the contrary. I see fasting more as a long term, life choice. I do feel more energetic and sharp during fasting.

I recently did a full check-up (I'm 48) and all my blood/urine values were spot on - before starting my fasting protocol some values mostly related to arteriosclerosis were slightly over the recommended values. Obviously, this doesn't say much or it's far from being a scientific proof: it's just an observation. There are good chances that for a lot of people fasting would just not help and make life misearable.