There is a more specific 5D3 version which doesn't contain stuff only found on the 1Dx but is otherwise almost identical. It explains things a lot better than the manual.
The 5DIII guidebook has been virtually copied from the 1DX guidebook and subsequently edited (carelessly). On page 5 of the 5DIII guidebook it gives an option "AF Status in viewfinder" which is available only in the 1DX and not on the 5DIII. I noticed a few more glitches but it is extremely helpful in understanding the AF.