Adding additional water to already simmering bone stock
I have never seen a stock recipe that recommended 3 days. In my experience, after 12 hours the bones gave up all they had and would crumble when touched.

Even so- simmering with the lid off will cause quite a bit of evaporation. Definitely add water whenever you notice it is low.

I've had stocks dry up completely and it smells absolutely horrible when they burn.