The first thing is to mark all the eggs under the hen when she starts so you don't end up with a staggered hatch - which can be disastrous. Remove all eggs other hens volunteer to the nest.
Sometimes it is better to have the sitting hen in her own quarters but it can be done among the flock.
Try not to disturb her for the 3 weeks. Wait till she takes her daily foray for food, water, defecation and dust bathing to remove the volunteer eggs.