I have always used a two day rule[not with fish]. My own daughter, recently. says she always uses the 3 day rule.
I thought I would try it last night. There was a leftover perfectly loooking good, 2 1/2 day old sausage in the fridge. I heated it up and ate it. No ill effects at all this morning.
I am now thinking of all the food that I have given to the cat over the years that may have been perfectly fine to eat myself.