My 60d has 3 options in the custom functions for selecting what focuys sceen I have installed. And the manual says if i change the focus screen to set the custom functions correctly. Does this mean my 60d has interchangeable focus screen and the 5d MkIII does not?
Correct. But your 60D does not have the advanced transmissive LCD either displaying the grid lines, focus points, AF modes, etc...
That is the tradeoff... Focus screen or awesome viewfinder information. I will take the advanced LCD!