When you say she has stiff legs, do you mean they are paralysed or partially paralysed? Perhaps stuck out in front or behind? At that age there is a good possibility that all 3 chickens are victims of Marek's disease. Paralysis is just one of many symptoms of Marek's, muscle wastage is another, which fits since you mention they are skinny. Visceral tumours from it can cause sudden death with no apparent symptoms, so the first two to die may have died from that, but as someone else has mentioned, a necropsy is the only real way to answer the question.
The best you can do for now is supportive care with good nutrition. A chicken sling or hammock will help to keep her in an upright position which will improve general well being and food and water pots can be incorporated into the design so that the bird has easy access. It also helps prevent them from soiling themselves and lying in it. If you search for images of chicken slings or chicken hammocks you should come up with plenty of pictures of both quite professional looking ones as well as home made efforts from everyday items like an old t shirt or tea towel and a cardboard box with yoghurt pots cable tied (zip tied) onto the side for food and water.
Good luck with her. I hope I am wrong about Marek's but supporting the immune system with vitamin supplementation, good nutrition and plenty of sunshine and grass and at least being able to see other chickens will help. If it is Marek's the rest of your flock have most likely already been exposed. I no longer isolate Marek's sick birds and I find that they are more likely to survive if they have companions, but within a safe environment like a cage, so that they can't get picked on. Don't get me wrong, it is still a deadly virus but I have had birds be pretty desperately debilitated with it and recover.
Best wishes
Barbara