If you can hear a rattling sound when the affected hens are breathing (pick them up and listen to their chest area) then I think it sounds like Infectious Bronchitis which can be brought in by other hens or by wild birds.
Unfortunately, whilst there is an available treatment, there is no guarantee that the birds will fully recover and the airborne IB virus has an area of effect of 1.5 miles around you, so there is a chance that if it is left without any action being taken that other chickens in the area can contract it.
Have you had the affected chickens for long and has there been new additions to your flock recently?
