The 5D III images appear to be quite sharp in Quick View (very similar to 5D II
image sharpness), but DPP is doing something wrong when processing them. This
issue has been replicated on a completely independent test (same camera, but
different memory card, different computer and different test subject/scenario).
Roger at LensRentals.com also appears to be seeing the same problem.
Canon has been contacted and will be working with me to resolve this
issue, but it will likely take some time. Shoot your 5D III images in RAW + JPG
and waiting until this problem is solved before processing the RAW files in DPP.
Or use another RAW conversion application