Hi Jen,
There is a lot of information on caring for broody hens already written in previous threads, do a search when you have time. Ideally, you need to separate the hen from the others, either by making a barricade around her or moving her (after dark) to her own private quarters. She'll just need room to get up and eat/drink/poop each day. It's not the best circumstances to have other hens going in to interrupt your broody, or to give your broody opportunity to get misdirected on her way back from her daily break and go to set in a different nest.
Mark the eggs she has under her now and remove daily any new ones placed there. Each egg begins to develop when the hen gives them constant moist heat and should hatch 21 days later. If new eggs are placed after others have already begun to develop then they will hatch later than the older eggs. This may be a problem in 3 weeks, the hen will want to take her older chicks out to eat & scratch and leave any unhatched eggs in the nest.
Another thing you can do is to try to move her into private quarters, leave her there for a day or two to make sure she'll stay there, and then remove all the eggs from under her and replace them with a fresh clutch of fertile eggs that will all hatch at the same time.