I am sorry Thelma is having trouble with her fur. She is considered geriatric now (8+ years old) and elderly cats can experience many natural changes and disease processes that can affect their skin and hair coats. The 3 major illnesses to afflict older kitties are Diabetes, Kidney Disease and Hyperthyroidism. The symptoms include vomiting, diarrhea, increased water consumption, increased urination, weight loss, changes in behavior, changes in appetite, vocalizations, muscle wasting, dehydration, weakness and poor hair coat. I understand that it is a regular occurance for Thelma to have some trouble with her coat, but one of the above illnesses can be responsible for making the usual trouble much worse. Miss Thelma will need to see the vet for some careful clipping and de-matting. If you are able it would also be a good idea to have a comprehensive blood and thyroid panel and a urinalysis run to screen for disease, and to evaluate organ function and hydration status. Older kitties have very thin skin, so get the matts removed as soon as you can. I have seen cats with badly torn skin from catching matts on objects. This is painful and can require antibiotics and pain medicines.
You are doing a great job with your kitties, it sounds like they are flourishing under your care. Your 2 other kitties would also benefit from some blood work as cats are masters at hiding illness.
I hope that Miss Thelma does well, and that your other kitties are happy and healthy. Let me know if I can be of further assistance, and good luck. If this has been helpful, please hit the green accept button.