If you’re here for info on installing Cyanogenmod on your S3 and just want to get to it, skip down the page a bit. First a bit of self-indulgent history, blogger-style:
Back at the start of 2013 my S3 phone started freezing randomly. My phone carrier, O2, refused to help me so I had to research the issue myself, a process which worked. I chronicled the mess in a blog post, including full instructions in case anybody else had the problem. You can find the blog post here.