Sprayer Pump on 3 way switch using Pixhawk Cube Orange Plus

Hello everyone, I am testing a spraying system controlled by the Pixhawk Cube Orange Plus Flight controller.
I have set up the following parameters:
I have configured SERVO9 to function 22(sprayer pump)
PWM range 1080-1950, Trim 1500.
I am using 3 way switch on RC to control the pump. I want to use sprayer low as off, sprayer middle as 50% of SPRAY_PUMP_RATE and sprayer high as 100% of SPRAY_PUMP_RATE. Right now I am able to control the pump using 3 way switch but at middle the pump is not working. I want that at sprayer middle the pump must be at 50% of SPRAY_PUMP_RATE. I am using a software where I can adjust the SPRAY_PUMP_RATE.
Firmware version: ArduCopter V4.4.2
Any ideas on how to achieve this, would be greatly appreciated.