I should add they are 7wks old and they like to sleep in a pile together on the floor.
Even when I had roost bars in the breeder (6" off the ground), they wanted to sleep on the floor.
Thanks everyone. I went out there at dusk and put one on the roost. Within a minute, the 2 darker ones flew up to it and stayed there while the other one jumped down.
Just checked on them and there are 4 up there now and the others are just piled atop the roo.