It has been almost three months since I sprained my ankle, and it's still swollen. What should I do?

Orthopaedic surgeon. You may have something else going on in your ankle such as instability (looseness), loose bodies, a cartilage lesion, or scar tissue that is causing swelling and flui accumulation within your ankle joint. Further or initial orthopaedic evaluation is warranted.
See a doctor. If you still have swelling three months after your injury, it is possible that you have a fracture in your ankle that has gone unrecognized. I would suggest seeing an orthopaedic doctor for appropriate x-rays and physical exam, .