Excellent pictures, thank you.
She's in the "I'm very sick" posture, her eyes look fine. Her comb is more pink than red, could mean sick, could mean out of lay. She's a little messy on the butt feathers, but not too much considering how poofy her feathers are there. Her vent appears dry, indicating she's not been laying, can you confirm? When she walks, is she kind of upright waddling, like a penguin or no?
She appears to be what I call a "production red" type of bird, bred specifically to lay lots of big beautiful dark cream to brown eggs (usually one a day) for a couple of years, and then not-so-much. They're fraught with reproductive disorders, often fatal. A three year old hen of such a breed is an old girl indeed, but some on the forums report girls of eight or more years. (I've never been so lucky with them, and have quit buying them.)
You didn't say if a veterinarian was a option.
I'm going to assume that due to covid and all of the many restrictions we're facing globally because of it, that a veterinarian is not an option at this time. So....
If this were my girl, I'd put her on amprolium (generic name, better known as Corid) in case she's suffering from coccidiosis. Cocci are an internal parasite that is found on all continents and in all soil around the world, there are several types of cocci, and an animal that is overburdened by this parasitic infestation doesn't always present with the same symptoms.
Cocci will take advantage of a young, old, or sick animal, attach themselves to the intestinal wall, and steal the nutrients that your girl needs to survive. The medicine works by blocking the B-vitamins (so don't offer vitamin supplements while treating her, but do offer them after the entire treatment is complete). This medicine will not harm your girl in any way, even if she's not suffering from cocci overload, so it's a safe measure to take and it's CHEAP.
You might consider worming her at the same time, different medicine but both can be added to her water at the same time. In my country, Piperazine is used for adult roundworms only. Ivermectin is a more complete dewormer, as it treats roundworms, threadworms, gapeworms and many external parasites. Ivermectin can be used orally (ingested) or topically (rubbed on their skin).
Albendazole also treats tapeworm, roundworm, capillary worms, cecal worms, & gapeworms.
Worming treatments require a second round of treatment 10 to 14 days after the first round is complete to kill off the hatching worm eggs before they reach sexual maturity (stopping the cycle of worm infestation due to the worms breeding).
Will keep you and your feathered family in my prayers.