I have never had one stay for 3 days, but probably what I should have said, is the next morning, they tend to be out of the old nest not an exact set of hours.
If you move before they hatch, do secure them in the place, I tried to follow advice and move my first broody hens to a place that I...
How flooded? Could you post some pictures.
I am going to tell you, I have never had luck moving a broody hen. However, every broody hen I have ever had, has always moved her nest in 12 hours of hatching.
So I would be tempted to let her hatch right where she is, and then once she has hatched...