Mareks in a 3 year old living with the same chickens she was with from hatch would be unusual. But time should,tell. The b complex can be crushed as well, and 1/4 to 1/2 tablet daily is plenty. Keep us posted on how she is doing. You could try her in a chicken sling chair if she will tolerate...
It won’t hurt to treat her for injury or a vitamin deficiency, and an infection or inflammation of an internal organ could also cause a problem. But she could have Mareks. If you don’t see progress and lose her, I would try to get a necropsy by your state vet to look for Mareks and other causes...