Hi
At 3 months with those symptoms, it is almost certainly Marek's.
If they continue to show an interest in food, I give Marek's sufferers supportive care and I have had some make significant, if not miraculous recoveries. Once they lose interest in food, I euthanize them. Diarrhoea is not a good sign in a Marek's infected bird as it often indicates an infection of the digestive tract. Mare's compromises the immune system and leaves infected birds open to secondary problems like coccidiosis, e-coli and respiratory disease. If you are emotionally attached, then making a her a chicken sling/hammock(google to find images of home made DIY slings) will keep her in a more upright and comfortable position and feeding her some good nutritious treats (scrambled egg, tuna, cat food and meal worms) and a vitamin supplement like Nutri Drench may help, but if you are not attached, it may be as well to cull.
Human vitamin B tablets (available in most supermarkets/large stores) can be crushed and used if you don't have anything else. B vitamins are reasonably safe to overdose so don't worry too much about that. Try a 1/4-1/2 a tablet crushed and sprinkled over whatever they will eat or dissolved in water. Giving them moist food is important so that they don't dehydrate and a probiotic may help with the diarrhoea. That can be a commercially produced product for animals from the feed store or some live/active natural yoghurt or by fermenting some chicken feed (which is really simple but takes a day or two, which you might not have.)
Good luck to you both whatever you decide.