Most hens are defensive when on the nest. Some will sit there most of the day.
A broody hens stays on the nest day and night, and cluck clucks when removed, and will often return immediately.
If your other hen left after 3 days it wasn't really broody yet. I will make sure they are sitting continuously for 5 days or more before giving them eggs to hatch, or just one day of it if I want to break them.