I had tibia plateau depression surgery 3 weeks ago, i am still not able to bend my knee beyond 45-60 degree. what do i do?

I would suggest two ideas that you should explore. Firstly, your operating doctor is the first person you should contact and ask for his/h advice to help you with your problem. Secondly most any type of surgery around or on the knee would likely require physical therapy(PT) to help move the joint into its maximum range of motion. So ask your surgeon about PT and follow his suggested line of action