If they are 5 years old, they died of natural causes. They died of old age. Get new and younger chickens, you don't have anything to worry about. If you had a virus that they were susceptible to, you would have wiped out the flock rapidly. Loosing 3 chickens over a couple of months is just more than likely old age.
Mrs K
Without laying a scientific study on you all, here's a very charming yet informative article abut hens and dying of old age and what actually ends up killing those that dare to keep living beyond the ripe old age of five.
https://hencam.com/henblog/2011/02/why-old-hens-die/
In my own flock, in addition to a fleet of younger layers and two roosters, I have several ancient hens, from age eight to eleven. A ten-year old keeps trying to lay eggs, but she hasn't got the chops to build a shell any longer, so she may end up dying from reproductive issues. Indeed, she almost died last month when one of her shell-less wonders ruptured inside her. She completely recovered with the help of penicillin and a week of intensive care.
The eleven-year old has a tumor over her right eye. No, it's not likely to kill her as she's had it half her life. But she could eventually die from leucosis tumors on heart or liver. So far, she shows no signs of being a candidate for death from "old age".
Even though I have an avian virus in my flock, I rarely find a chicken dead from "mysterious" causes. If one dies, which is rare, she has shown signs of steady decline. More often, a hen or rooster will get sick and go through a slow decline, and I euthanize when they no longer appear to be enjoying life. I do my own necropsies these days and often find a body cavity loaded with tumors.
Saying a chicken has died from natural causes or old age usually assumes something killed them. Most often it's tumors that finally cause organ failure.
