What makes you say that?
From what I understand, the capability of the 5d3's af are depend heavily on the max. open aperture, or more and more af points loose part of the pattern detection - so if using say a 70-300L/4-5.6 you've got part of your af system disabled @300mm. It's ok to design it this way, and many promotion shots show the 5d3 with the 24-70ii or the 70-200/2.8.
On lesser bodies like the 60d, there are "sensitive" af points meaning that they control the focus more precisely, but the pattern (cross, double cross, ...) is independent meaning the manufacturer expects this af system to be used mainly with slower lenses.