Treatment for grade 3 sprains should firstly include RICE (rest, ice, compress, elevate). Ice should be used for 15 minutes at a time and repeated every 3 hours throughour waking hours. Most people make the mistake of using it once or twice and thats it. You need to keep this up for at least 48 hours.
You need to start range of motion exercises firstly (like writing your name in the air with your toes) before starting gentle strengthening, like with a resistance (rehab) band.
You can find more info about sprained ankle here