I got my 5D3 last Friday and took few hundred photos with no issues. Today I took ~200 photos and 3 of them in a row were corrupted. Lightroom 4.1 could read 2 of the 3 but chunks of the photos were missing. It wouldn't read the 3rd.
DPP could read them but they were blocky/pixellated (like they had been reduced to a small size and then blown up). It could not export them to jpg.
This was with firmware v1.1.3
Attached is an example of the jpg output from Lightroom 4.1 and a crop of what Canon DPP shows me for the same file.
It sounds like corrupted download, did you use a card reader? Try downloading them from the camera.
The image is blocky because DPP is showing the tiny jpeg thumbnail that is embeded in every raw image, but the main image is corrupted.
The usual cause of this is a bad card or a error in downloading using a card reader.
Try it from the camera, do a chkdisk on the card to check for bad sectors, try a different card reader or at least a different USB cable.