What transmitter do you have? Likely, you won't be able to have 3-position flaps when you use the flaperon mixer. That's because you are using the surfaces as dual-purpose surfaces: ailerons and flaps. You need to have them centered for most flying, but then want them to deflect downward when you flip the flap switch. Unfortunately, the mixer will see the switch as +100%, then neutral, then -100% when you move it through its three positions, so you'd get flaperons down, flaperons centered, flaperons up.

In order to actually have multi-positioned flaps, you'll either have to use a knob to set the flaperons, with the attendant difficulty of finding the center while in flight, or go to separate flaps.

If you have an advanced transmitter, such as a 12FG, 12Z, or 14MZ, then you'd use the Camber Mix to set your positions, and use conditions for each desired flap position.