There are a number of things that can cause a hen to go off lay. It's generally the wrong time of the year for mounting, but not unheard of. The dusting your seeing is normal behaviour for healthy chickens, but also accompanies moulting. If she's not losing feathers and doesn't appear scruffy, it's probalby not that. Secondly, illness in chickens is quickly determined by the color and "uprightness" (for lack of a better word) of their comb. Should be bright red and not folded over or limp. If it does indicate a problem, my first suspicion is usually Coccidiosis, since it's a parasite induced illnes common in chickens. It's treated (once diagnosed) with various sulfa type drugs available form a vet or supply store and fed with their normal feed. Keeping their entire area clean helps in controlling the outbreaks. If the other 2 are healthy this is probalby not indicated... Best of luck!