It sounds as if they have gone broody, Trerecoco. In other words, they have developed the hormonal condition that makes them want to sit on eggs and hatch chicks. If you don't intend to let them do this, you need to break them of the habit, because just sitting in the nestbox, with no exercise and very little food. Will be bad for their health and of course they won't lay eggs whilst broody.
You must get them out of the coop and not allow them back in until they are cured. They need conditions where they haven't got anything comfortable to snuggle down into, and where they can get light and exercise. When my Sussex goes broody I sweep the bedding away from a corner of the run and make her a small enclosure, fenced off with netting so she can see the other hens but not get out, with food and water of course. Then she stays in there day and night, for about three days usually. She spends a lot of time marching around making indignant comments, so she gets air, exercise and light. After three days I try letting her out with the others again, and watch t see if she goes back in the nestbox. If she does, I put her back in her little run for another day. If she doesn't, she is cured and an rejoin the flock, though she won't lay again for a week or two.
With my bird, it usually takes three days because I've got to knowing the signs and I treat her as soon as I see what's happening. But if yours have been like it for 3weeks and have some Silkie in them, it may be harder for you to shift them, and it may take longer. Silkies are renowned as good broodies and do it often and are very determined! Some people put them in a wire dog carrier, especially at night if its not safe to just leave them penned in the run, so they get plenty of air below them and can't perch comfortably. I think the main thing to remember is that this is a hormonal condition which can be eased in time, with humane treatment, but you have to be determined to help your birds get over it, or in some cases they will just waste away from useless sitting.