I'd put them under her tonight after it is dark and check first thing in the morning. They should be fine overnight, I personally would not worry. But check early tomorrow when she can see. As Kiki said, you never know what they will do but I feel pretty good about your chances.
I'd try the other hen first, I think your odds are better. Some people believe a hen has to set 21 days before she accepts chicks. That's not been my experience, I had a hen accept chicks two days after starting being broody. But I do think your odds are better if she has been broody for a while. Also, not all of my hens are broody enough to deserve eggs, let alone chicks, when they first go broody. With some it hits like a ton of bricks but with some they have to work their way into it.
You can always try that second hen if the first one fails. It might work, it might not. You never know what will work.
I often have eggs hatch two full days early under a broody hen, about the opposite of your experience. Even under a broody that 21 day thing is just a general target, not a sure thing. That's why many of us suggest patience before you do anything dramatic. Next time you'll know.
But were you counting the days correctly? An egg does not have 24 hours' worth of development immediately after it goes under the broody. It takes 24 hours for it to have a day's worth of development and you can say "1" when you start counting. An easy way to check your counting is that the day of the week you set them is the day of the week that 21 days are up. If you start them on a Friday the 21 days are up on a Friday.
That's a real common error on here, happens all the time. Your three were still late, even if you did this, but maybe not quite as late as you thought. Anyway, good luck!
