Sure, she'll give it up. Three weeks is the usual hatching time. They seem to stay about an extra week and a half around here if the chicks don't hatch. So if it doesn't get colder, then she may keep brooding for about 4 1/2 weeks--all the while keeping your other eggs from freezing. If it gets colder, she may come off sooner. It depends on the hen.
She's actually a slave to her hormones. When her hormones take a dip is when she'll come off the nest and then usually start laying again in 10-14 days.
All my hens have eaten when they are broody. Have I seen them all eat? No, but I've seen a few. No hen can go for three weeks without eating. Her hunger will get her up off the nest. You just have to have food out all day long for her and take water out at least two times if it's freezing in the coop so she can have some to drink in the late afternoon if she missed getting water before it froze after you took it out in the morning. Sometimes mine don't get up to poop and just do it on the back of the nest and shove it to the back of the nest. If she's pooping, she's eating.
When there is another laying nest available and the hens won't use it and they squish all together, I think, "Well, I'm sure that's some behavior I haven't read about yet that is totally normal in the wild or in their instincts somehow." I guess, if they didn't like the situation, they would move to the other nest or quit laying. I saw a hen in my coop yesterday lay an egg right on top of another hen's head. 9 nesting boxes, 2 in use and one of the hens insists on squeezing into a nest and laying an egg on top of another hen??? Maybe they do that because they know it's a safe nest if another hen is laying there??
Hens are very intelligent. Feel free to watch your little gal, but I've never lost a broody hen (about 30 hatches at least) while on the nest to starvation.