Unfortunately the focus screens on the 5D Mark III are not replaceable.
Seriously? But 6D can do it? So silly since i think 5d mk3 has more "pro users" who would want to change the focus screen.
The 5D2 had it, but for the 5D3 it's one of the trade-offs for getting the 61-point auto-focus of the 1D X, which allows Canon to differentiate the camera lines. So, if you only want the better auto-focus system, get the 5D3, if you only want the interchangeable screens, get the 6D or a 5D2, but if you want both, you have to get the 1D X (or a 5D2 and 5D3).