I have a history of failed fusion. I am 3 months out from last fusion &anxious to end this never ending pain. How soon can I start pt? Any other advic

Now. You should be able to start therapy now to help facilitate your recovery. You should check with your surgeon to see how aggressive the therapists can be with you. If you are still having pain you may want to consider a spinal cord stimulator - it often provides tremendous relief for people in your situation.
PT can be started. Continued pain after a fusion has to be tested with the following question: was the wrong operation done or was the right operation not done correctly?. That being said, you can still improve. Physical therapy typically is the potent tool for improving back pain in this setting. Postoperative x-rays will determine how aggressive you can be, but there is always some type of therapy to utilize.