This is what you do: Get fresh chicks, takes them down at dusk but do not put them in with her. They need to get a little chilled, so that they are peeping madly. The peeping helps get the broody ready to make the switch from setting to being an active mother. An hour or so after dark, slip down there with a flashlight, but with as little light as possible. Stick the cold chicks on her back, or directly under her. Wear a long sleeve sweatshirt in case she pecks at you.
Those cold chicks should burrow in tight as ticks. That movement under her also helps trigger the right hormones, the chicks should become mostly silent as they warm up, and she should start talking to them. She may peck at them, but ignore that. Keep it dark, and get them under her and then the hardest part... leave her alone till morning, and even then do not "help" her. She knows more about being a chicken than we do.
Good luck, I would add one more than you think you want. Sometimes chicks die.
Mrs K