Canon EOS 5D Mark III Instruction Manual page 367

There are fewer AF points.
Depending on the attached lens, the number of usable AF points and
patterns will differ. The lenses are categorized into eight groups from A
to H. Check which group your lens belongs to. Using a lens in Groups
F to H will have fewer usable AF points (p.79).
The AF point is blinking.
When you press the <S> button, the blinking AF points are those that
are not cross-type AF points and are only horizontal line-sensitive. The
AF points that light up are cross-type AF points (p.74).
The AF point at the registered area is blinking (p.330).
I cannot lock the focus and recompose the shot.
Set the AF mode to One-Shot AF. Focus lock is not possible in the AI
Servo AF and AI Focus AF modes (p.70).
The continuous shooting speed is slow.
Depending on the shutter speed, aperture, subject conditions,
brightness, etc., the continuous shooting speed may become slower.
The maximum burst during continuous shooting is lower.
If you shoot something that has fine detail (such as a field of grass),
the file size will be larger and the actual maximum burst may be lower
than the number mentioned on page 123.
ISO 100 cannot be set. ISO speed expansion cannot be selected.
If [z3: Highlight tone priority] is set to [Enable], the settable ISO
speed range will be ISO 200-25600 (or up to 12800 for movie
shooting). Even if you expand the settable ISO speed range in [ISO
speed range], you cannot set expanded ISO speeds (L, H, H1, H2).
When [z3: Highlight tone priority] is set to [Disable], ISO 100/125/
160 can be set (p.148).
Troubleshooting Guide
367