there are a few things I know that can cause rear leg problems, impaction, MBD, injury to spine or neurological disorders.
Impaction can be due to feeding too large prey items. For adult dragons, this typically isn't a concern as the bulk of their diets are greens. However large hard pieces of stems, etc from greens could cause blockage. Other items that can cause impactions are loose substrates (any 'calcium' based sands, crushed walnut, wood shavings of any type, etc).
Improper amounts of calcium in the diet, or exposure to UVB light can lead to metabolic bone disease.
the above two conditions can be determined by a vet by taking blood work for tests and x-rays. Blood work can show calcuim levels and other useful data. X-rays can show blockage and bone density.
blows to the lower back from cage furnature falling on a dragon can also cause partial leg paralysis or complete paralysis, depending on the severety. Again a vet trip with x-rays to check for spinal injuries or inflamation is needed.
Old age can of course, cause this as well but a 3 year old dragon is no where near being old. I know you indicated you ahd the dragon for two and a half years but not sure if you bought her as an adult or baby. Dragons on average live about 6-10 yearss, though some can live as much as 15 years.
More details on your cage setup, your normal daily routine for food/care etc and what the vet did when you brought your dragon in to see him will help to pinpoint what could be wrong.
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PHLdyPayne