What's killing my chickens?

I got 7 ex-factory farm hens last summer. According to the charity that buys and re-homes them, they should have been about 15 months when we got them. So they are about 2.5 years old now. Over the past couple of months, three of them have died from some illness or parasite or something. Symptoms: lethargy, floppy comb, tail tucked down. At the moment, three of the remaining hens are still perky, but one is exhibiting these symptoms. I had said that I would take her to the vet if it happened to another chicken, but I find that my finances can't stretch to a vet's visit this month.

Also, we have not had any egg-laying to speak of since early summer; for the first 9 months or so, they were giving us 4-7 eggs a day, but after that it dropped steadily and since the summer, we're lucky if we got 1 a day: even when all 7 were still alive we still only got 1 or none every day.

The hens are in a paddock system in our small back garden, with access to grass and some veg and ornamental beds. They also get pellets and kitchen scraps.

Any ideas would be appreciated!

If it took a couple of months to kill them, it's most likely not a disease. You should probably worm them.
Also add a bit of apple cider vinegar to their water, as it helps keep their immunity up and encourages gut bacteria to grow after the worms are killed.

Thanks for the suggestion, which I will definitely try; though I probably should have mentioned that they only exhibited symptoms for about a week before dying.

Luckily the fourth one is still alive and looking a bit better. No more floppy comb, at least, and not so lethargic.

What breed are they ? The super-hybridized production breeds like pearl leghorns for egg farms might wear out after two years . The fact that they died in close succession with the same symptoms points toward a contagion or parasite . Especially if they are a longer lived breed. Did they exhibit any sneezing , coarse breath sounds , mucous drainage . Did the combs turn color - blue or purple tinted ? What was the condition of their feathers ? Had they lost weight recently ? Any decline in appetite ?

I was wondering if it had to do with their breed; I don't know what breed they are, but I'm certain they are some industry-standard hybrid.

For the other symptoms: as far as I could tell, there were no breathing problems, but one had a small white patch on her comb (I'd assumed she'd been pecked there). The first one to die was quite thin when she died, but the other two were still big and heavy, and all their feathers looked normal. I think they were all still eating until the very last day, though I can't be sure.

Don't forget, factory hens are typically fed antibiotics with each meal.
This prevents their own bodies from developing its own immune system.

They were probably housed, 2 hens to a 2'x2'x2' cage. Their feet never touched the ground.
They lived their careers in a controlled environment.
Relocating them to the 'real world' is a shock that their bodies had not been acclimated to.

Factory hens are not known to do well, and last long in the real world.
The industry uses artificial lighting to keep them laying all year.
When the factory gets rid of them, they are known as "spent hens".