3 months is the prime time for Marek's disease to manifest. I personally do not euthanize Marek's birds unless they lose interest in food and I've given some supportive care for months during which they have improved enough to return to free ranging with the flock.
You could make a simple DIY chicken sling/hammock to support her in an upright position and fix empty yoghurt pots for food and water to the framework with cable ties (zip ties) so that she can reach food and water easily through the day when you can't be there and is not soiling herself and lying in it. Getting her out into the sunshine for supervised fresh air and a bit of grass and being able to see other chickens and eat with them, albeit on the other side of a fence will help to stimulate her and keep her from getting depressed. I have had birds make significant if not miraculous recoveries from quite serious Marek's attacks where they were floundering on the ground unable to stand. Unfortunately, some deteriorate and die and those that do recover, will be prone to further and usually more serious attacks but that could be months if not years later, so I give them the chance to fight it as long as they are bright eyed and eating well. Supporting her immune system with vitamin supplements and good nutrition will also help. Of course you may just decide to end it for her now, but just wanted you to know that there is some hope and not all your birds will get sick with it. I've had it in my flock for 3 years and I lose maybe 3 or 4 to it each year but if you consider that my broody hens raised 56 chicks last year, that is not a really significant figure, although each individual loss is heart breaking.
I take the view that my flock has already been exposed to the virus, so I keep sick birds within the flock or in sight of it as much as possible, otherwise they get depressed and give up.
Good luck with your girl and the rest of your flock.
Regards
Barbara