I have read couple similar cases for 5d3 + 85L. I have the same issue for 5D3 with 50L. I reported in another thread http://www.canonrumors.com/forum/index.php?topic=11969.0
So if it is convenient for you, you can try Canon Service center and let us know the result. Since I need to spend nearly a whole day drive back and forth there, I have not tried them. I called though.
I researched another forum and read couple other people's experiences and talked to Canon Service center. 5D3 has micro adjusted out side focus points (not center focus point) in order to fix those F1.2 lens back focus issue. This is confirmed with Canon Service center.
So my conclusion is if the lens itself has back focus issue, it will be fixed on 5D3 and perfect. If the lens has no back focus, you will get front focus on 5D3. And as well as 1Dx. The only fix is if Canon Service center would spend time to adjust your camera setting with your lens. It is a case by case issue and it does take time. So that is the reason I am afraid I would waste my time with the service center.
So I would suggest: 1. return the lens if possible. 2. try more copies if you can. people do report all accurate focus points with your combination. 3. use center focus point and crop. 4. try Canon Service center.
After spending good money and expectation, I can feel your pain. Take care.