My concept is from 3PKS, hope is the same setting,
first of all, you need to know what is the name of this botton, like DT1 or DL1, if you can confirm the name.
Go to menu and look for the FUNC-SEL-Dial, then select the correct name of this botton and set it to channel 3.
I hope its the same setting of 3pks!!
Good luck!