What's the likely hood of bonegraft failure after 3 week post op check positive. dentist advises i can chew on anything. i am healthy 22 years male.

Low: If all was well 3 weeks post-op, normal healing was established and presumably no membrane slippage had taken place by then. It takes awhile for normal healing to proceed to the point where you are ready for implant placement (if that's why you had the graft). Are you having symptoms ? Have you seen something worrisome? Need more info to answer your question well..