I have a canon 5D mkiii. Previously I had the mkii and it worked fine, so my current software needs upgraded to support the mkiii but it won't install. I have CS5.5 PS. Now I'm having issues with LR, as it won't allow me to move files from LR to PS and that's how I was working previously when the update didn't take.
The Canon 5D Mark III requires at least ACR 7.1, which requires at least Photoshop CS6 which is no longer available for purchase. To use ACR and Photoshop with your Canon 5D Mark III you will need to do one of the following as previously suggested.
1. Upgrade to Photoshop CC 2017 (Requires a Creative Cloud subscription).
2. Download and install the free Adobe DNG Converter (see my previous message) to convert your images to DNG before opening in Photoshop CS 5.5.
The version of Lightroom you have may be using a more current version of ACR internally which is incompatible with the version of ACR in Photoshop CS 5.5.