They need access to DRY fine dirt. If they dont have that, I would put a large (2 feet by 2 feet at least) tub with some fine dirt/dust in their coop for a few hours a day. In a sunny spot if possible. My girls get along pretty well, but if you have your 3 in tight quarters they may get bossy over the dirt bath, food and roost space.
You can get a black concrete mixing tub from Home Depot or Lowe's and put some sand in there (also DE is good, food grade), ashes from your woodstove when you have them, and dirt (or just sand in there).
But I agree with gritsar in that they will dig some holes and enjoy the dirt. If you get a lot of mud you might want to cover part of the run so they have a dry spot for a bath.
I have a 4'x4' box made from railroad ties filled with half play sand and half peat moss. It's a big hit with my hens, but they still love to create their own dirt bathing holes all over the place. Since chickens like to clump tightly together when dirt-bathing, it doesn't require a very large space.