What's the holdup on getting a 300 Euro PC laptop and a 30 bucks a month Creative Cloud? You were able to buy a 3000 Euro camera body, were you not? I know this sounds rude, but it really is pretty ridiculous on some level...
Here's the bottom line:
You will never be able to open the image files from your Canon 5d mark III directly in CS3, because that camera model was first supported in Camera Raw 6.7 (which only works with CS5) and 7.1 which only works with CS6.
However, you can download the free Adobe DNG Converter 7.1 and convert your Canon raw files to raw DNGs that you can open with any version of ACR starting with ACR 2.4, which takes you all the way back to CS1. Therefore, you'll be able to open them in CS3 with no problem.
Get the latest version of the Adobe DNG Converter, it is a stand-alone application that is not tied to whatever version of Photoshop you happen to use. The current version is 7.1 and there's also a beta version 7.2RC1.
Yes, if you can afford a Canon 5D Mark III, you can afford to upgrade from CS3 to CS6 for very little money ($199 for a perpetual license) if you act fast. CS3 will be out of the upgrade loop in late December or so.
I'm one of the many to whom the prospect of buying into Adobe's subscription mode and end up paying the Adobe tribute every month for life seems nothing less than repulsive.
With that camera (a rich man's camera) is silly to be using a laptop (a beggar's machine). Get a decent desktop Mac or PC already!