Im 22 hours into a 72 hour fast, my longest is 48. I make sure I’m getting electrolytes by drinking Smart Water and taking a electrolyte pill. I love flavored sparkling water, I love pineapple Bubbly. Good luck!
You should definitely add electrolytes. Magnesium in pill/capsule form (once at night is fine), and salt and potassium during the day.
Be very attentive to how you’re feeling during running/walking/strength training on a three day fast.
I speak from experience - I’m not super fond of the taste of electrolytes, but on any fast over 24 hours without them I suffer, and you can’t really catch up by adding them after your start to have negative repercussions. Common symptoms include weakness, feeling cold, tired, getting headaches, feeling dizzy, heart palpitations and potentially fainting.