A chip fracture of the lateral malleolus is usually treated as a lateral ankle sprain. As per the doctor's prescription you had already been immobilized for it for a period of 3 weeks which is usually sufficient for ankle sprains. Post that period balancing exercises and muscle strengthening exercises along with ankle mobilization exercises are advised to regain the balance on the ankle. So Physiotherapy has been advised at the right time. You should however avoid walking on uneven surfaces for a further period of 3 to 4 weeks.