First: the Zero delay encoders aren't keyboard encoders, but gamepad encoders. Second: why use the third encoder while the select button on P1 and P2 actually are used as the coin buttons? Third: the Zero delay encoders will have to have all buttons attached to the same inputs on the board as far as I know.
So, read my second point again and just use two encoders and attach the coin door as well as the secret coin button both to the P1 select input. Some games use two different coin buttons though, so if you only have one secret coin buttons and one coin acceptor, you could wire the coin door to P2 select and the coin button to P1 select (or the other way around). I believe when just one coin button is used in mame, both P1 and P2 select will work for coins. When two coin buttons are used, you can use the secret coin button for P1 and the coin door for P2 (or the other way around).