I have two F3HP with different focusing screens. One has a P screen, which is exactly the same as the standard K but with diagonal split instead of horizontal. The other has an E screen, whose use can be described by what kitanikon said above - it lets me focus pretty much anywhere inside the frame without getting distracted by the split prism in the middle. P is for when I need critical focus and want it confirmed and E is used in all other situations.
I do still have good eyes (I am 43) so can't really say anything about how all this works with glasses, but there you go.
Here are different focusing screens for F3, might be helpful