What should i do with a (grade iii) ankle sprain?

Not much: Often times need surgery and if not strict immobilization with non-weight bearing.

Many possibilities: R.I.C.E. Rest, ice, compression, elevation. Immobilization with a splint or brace may be of benefit in the initial phase of recovery. Oral anti-inflammatory medication may help with the pain and swelling. Once improved, mobilization and physical therapy can help restore strength and function. Healing times may vary. Have it evaluated by a professional and get the appropriate treatment.