its not included in latest firmware, they fix the issue on 1DX, but not on 5D3, here's quote from
petapixel:
Here’s what Craig Blair over at Canon Rumors was told by a source regarding this:
They are testing that at the moment but don’t know if it is possible to implement. The former system had a different type of focusing screen and it was easy to handle because it had no reflection in the viewfinder. The new system, with the LCD screen inside the viewfinder is reflecting the red light back and this can affect the light metering system. In the EOS-1D X, the viewfinder box is bigger than in EOS 5D Mark III, which aids in making it possible and workable. In the EOS-1D X they alternate red illumination and metering, and this is why there is the red blinking. With the EOS 5D Mark III, they are testing it but they are not sure if they can make it operate in the same way.
and yes im refering the top right button on the back of the camera, the other way you can press a centre joystick as well, and then press the centre one more time. (1st center is to make a center point, and 2nd centre press is back to previous point)
try to use AF point, the one square with a point at the centre, you can activate it by pressing m-fn button, but you need to set the button to do the AF point selection first in the settings menu.
yes it is, but it cannot stay red just like 5D and 1d series, they use a different methodology to do this, the old one is using led if I'm not wrong, its easy to handle, not with this new type.
most of the people notice this are from old 5d users or from 1D series users.