Have you wormed them, the flock? When, with what? You say three months past the point of laying, that would make them about 8 months or so, right? The entire flock is this age?
You can search EACH symptom that the one or ones you lost have shown. If they have never been wormed and if they free range especially, it is time to worm the flock. Search "Worming with Valbazen" here on BYC. Note posts by dawg53 for info on the worming, on Valbazen, the oral dosage, etc.
A regularly cleaned coop, roosts, run -- good food (layer if you have them for eggs)... check them for mites. Clean water -- Apple Cider Vinegar added to the water helps keep it from getting slimy in the waterer.
Hay or PINE shavings in the laying boxes, nests.
I would worm them (if you haven't) right away. You will need to repeat the wormer in ten days and withhold eggs for the whole time to the 15th day after the last worming.
Get Valbazen at the feed store and syringes (without needles) as well... so that you can dose orally, each chicken. 1/2 cc/ml per standard chicken. 1/4 cc/ml per bantam or small chicken. Worms can cause a variety of symptoms, including limping and death.
Good luck.