What are good exercises do to strengthen previous sports injury (on knee, ankle, calf)?

Resistant Band exerc: A resistance band (often also called theraband or rehab band) is a stretchy length of latex which can be used to provide resistance to most exercises. These have proved to be especially popular following injury.

Here are some: Straight leg raises wall squats single leg squat (only 45 degrees) calf raises and calf drops from a step a formal session with a physical therapist could be worth while to get you on your way previous ankle sprains need specific treatment to improve range and proprioception.