What is the most advanced version of ACR that can be used with Photoshop CS5?
Will it open RAW files from a Canon 5D Mark III ?
CS5 can use up to ACR 6.7.1. CS6 started with ACR 7.0.
The 5D Mark III was first supported with ACR 6.7.
ACR 6.7.1 can be downloaded from the Camera Raw Mac and Win links on the Adobe Download page:
Photoshop CS5 can use Camera Raw 6.7. That was the first version of Camera Raw that supported your camera. But I believe it was preliminary support that was updated in Camera Raw 7.1. I'm not sure that because I'm not really familiar with the Canon models. But if you use a later version of the DNG converter to create DNG copies that would work for you.