diverters and showers

[Gail]

Hi,

I have a kohler shower with both a handheld and regular shower. The diverter was incorrectly placed in the shoer arm. To correct this, I need to palce a mixer value in the wall and then a new curved shower arm on the regular shower. How do I know which wall mount transfer valve trim and valve to use? Can you use a 3-way valve even if I only need 2?