I set my F3 up on a tripod and did some experimenting. Focus remains the same with both F2 and f3 screens.
My F3 screen went into my F2 no problem when it was put in lip forward.
I have to be honest with you though, to me the original K screen is the easiest to focus with compaired to an H2. I found the H2 screen a disappointment as I was expecting something similar to the full microprism screens on my Leicaflex SL's but it's not.
There is a pin at the front of the F3 screen bay which is pushed in when an F2 screen is used. I have no idea what this pin does as it does stay out when an F3 screen is used as there is a recess for it in the front of the frame. If you are really keen on an H2 screen I would just get an F2 type, try it as is and then if you really like it then swap it over into the F3 frame.