Below link is in Norwegian and contains mk 2 vs mk 3 image comparisons with EF 85mm f/1.2L II. To see the images, click on the respective grey line. First appearing image will be mk 3, if you point on the image, corresponding mk 2 image shows...
...and here is their conclusion at the end of that article (from google translate):
As for picture quality, it is to launch a successor to the 5D Mark II in many ways to jump after Wirkola. Just picture quality was 5D Mark II's strongest field, and it is the age, despite still some of the sharp one in the market when it comes to this. When Canon still managed to give the 5D Mark II one of the most noticeably improved image quality, it is somewhat impressive, although we still are a bit between satisfaction. For although the increase is there, it is less than we wish. When Canon has chosen to barely increase the resolution, one should be allowed to have aspirations for a greater increase in overall image quality than this. Detail reproduction is good, and good noise level, while the noise is very very smooth. The noise level could, however, with advantage have been even lower, and this would also done wonders for dynamic range, which ends up being disappointing stuff.
All in all, we are cautiously conclude that although the image quality is improved in most areas, this is not in itself a good enough reason to replace the Mark II to buy the Mark III. There are all the other improvements in speed, auto focus, control, build quality and features and capabilities, which should be the decisive factor for it. If not, keep your beloved 5D Mark II a few years.
If this holds water when it finished the camera, fully supported by various software, comes on the market, we will of course come back to a full test.