I have done long fasts and also played with intermittent fasting for a couple of months. For me, I found that 3 days was too long - I started to have brain fog quite often and would have a hard time functioning. I found that a maximum of two days worked best for me and I liked to keep it pretty random with the eating and the fasting so that my body wouldn't get used to it (sometimes 1 day of eating, then 2 days of fasting, then 2 days of eating, 1 1/2 days of fasting, then 1 day of eating, etc). It worked fairly well for me.
Now I would only fast for a maximum of 2 days or at least 7 or longer. But you will see what works for you.