For inside the coop, take a milk crate and place it upside down over the chicks the first night. During the day, elevate the crate a couple of inches on maybe bricks. Tape it or secure it to the bricks. This will allow a safe zone that the chicks can escape to, but the hen can not get them. Have a bit of food and water in there. I suggest using nipple water so as to minimize the water mess in the coop.
Switch your laying hen to the chick feed with oyster shell on the side. Eventually when the chick feed is gone, go to flock raiser, and by the time you are through that everyone can go on layer feed again.
3 week old chicks really do not need heat, but rather a huddle box. A rubbermaid tote box on its side will provide a good windbreak, with either a wooly hen or the mama heating pad in it for the day time. You could tack up a little bit of chicken wire to keep them close by for a few days. I do think chicks need fresh air and sunshine as soon as possible.
Mrs K