Exercises should have started earlier.Improvement could be gradual

Do warm water gargles two to three times in a day. Do warm application from out side too. This will help you in relaxing your muscles. Imeediately after this put icecream sticks in your mouth whatever maximum mumber possible. Then insert few more sticks between previous one,one after another. Do it for few days two to three times in a day

Jaw movement restriction can happen after the extraction.This may last for few days to few weeks.

If u would had opened ur mouth after 24hrs of extraction there wont have been any problem Now jus take Muscle relaxant Heat application And Mouth opening exercise It ll open again

this is due to muscle spasm due to inflammation of muscles responsible for opening and closing mouth. in addition to muscle relaxants, you should gargle with warm water to reduce inflammation in muscles and mouth opening execises.

Stiffness of the jaw muscles may cause difficulty in opening your mouth for few days post surgery. But as you said its already 3 weeks..now you have to take proper care , take medicines amd do exercises.

Practice excersise. you should have started earlier not after 3weeks.