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.

I found my first fast to not be representative of later fasts. You get more accustomed to not eating after you repeat the practice. You become conscious of how often you wander into the kitchen and stick your nose in the refrigerator and the pantry. A cold glass of water becomes a delightful snack.

You might try reducing your fasts to just 24 or 36 hours on weekends.

Currently, I do intermittent fasting where I don't eat before noon or after 6pm, 7 days a week. It helps me to manage my weight, I feel consistently energetic (I work out most days, cardio and weights), my mind feels as clear as ever, and I have no trouble following the practice as a lifestyle.

I'd say that I don't feel quite that clarity of thought when meditating after fasting for 30 hours or so. But it strikes the right balance of achieving some benefits of fasting without feeling like a huge chore that I have to overcome.

I had a period a few years back where I did fasts up to 3 days quite frequently.. Once I got used to it they felt pretty good. But, the point I wanted to add is that they would have been very difficult for me if I wasn't also doing a pretty strict low-carb ketosis diet. Without the hunger control of keto and having my body acclimated to using stored fat as a primary energy source, fasts have been untenable.

Did you supplement with any vitamins or electrolytes? I've done a few 100hr fasts and definitely experienced a decrease in physical energy, but my mental clarity was always improved. I have ADD and use a mix of intermittent fasting and alternate-day fasting as one form of my treatment.

Did you try it more than just for that three days (as in, try multiple fasts over time), and or try other variations of it (eg 16/8 intermittent fasting) to see if one of those might suit you better?