3 roos for 18 hens is 2 too many roos. If you're keeping the boys for breeding, try separating the birds into breeding trios. OR separate the boys and girls so the girls can get some of the feathers back. I have found that hens seem more relaxed with each other when there isn't a roo around.
According to the Mississippi State University site...
"Birds that frequently mate may have an absence of feathers, especially on the backs and heads of hens. The males may also have feathers missing from the breast area. These feathers will grow back after the breeding season is completed."
"If feathers are missing from the abdominal and vent area, the cause is most likely the presence of external parasites such as the northern fowl mite or poultry lice. Infestations of these pests can be controlled by regular sprayings of an approved pesticide like permethrin to the birds. The house and other structures that the birds frequently visit should also be sprayed. This will ensure the elimination of any pests that can reinfest the birds. Several applications at 2 to 3 week intervals will kill pests that hatch from eggs that have been deposited prior to the initial spraying."
