First thing, go see a physiotherapist asap. Ignore any advice that tell u to do.other forms of exercise or taking supplements until u see a physiotherapist as three weeks is too.long

With luck, it will be nothing severe & will heal.with proper treatment over timebut in the meantime do not put any unnecessary strain on your body with any form of exercise without consulting a physiotherapist first.

When u have healed, go see a proper trainer to devise a proper workout routine & do not follow YouTube videos as u may risk exacerbating your injury. Let the trainer know about your ankle/knee injury.