If your doctor doesn't know what the problem is, he can't "fix it." There are many conditions that can't be fixed.
It sounds to me that you had severe knee trauma 3 years ago, and have had total reconstructive surgery. When the doc said you'll always have a bad knee, he wasn't kidding. Now you're beginning to understand what that means.
It is important now to treat the pain. Perhaps you need a different medication or more of the same -- work that out with your doc.
Steroids can do a lot of good -- they reduce inflammation, which is part of what's wrong. Inflammation causes pain. Perhaps if when inflammation is reduced, Xray/MRI will be able to show what is wrong.
Physical therapy doesn't work in three weeks. You have to be patient, and you have to exercise as instructed every day.
Is your activity level such that you've reinjured your knee -- since you are again suffering pain and inflammation, you may need to return to resting and elevating your leg, applying ice, along with wearing an elastic bandage. Anti-inflammatory medications may also help.
Talk to your doctor about the pain. When pain and inflammation are under control, perhaps your docs will then know what's wrong and be able to offer treatment options.
My doctor said if physio didn't work in 2 weeks to leave because I had runners knee he said. And he said he wasn't sure it was just an experiment. And I actually stayed an extra week in hopes it would work.
Your doc is certainly a better judge of what's right for your knee than I am, so follow his advice. Keep pressing for answers. You should be aware of his treatment plan for you.