I use Jeti, but it should work with almost any receiver. On your MC32 you can check each channel on a menu called "Servoanzeige" (in English it must be something like "Servo display"). When you hit your 3-way button, there must be one channel that changes when hitting the switch. That is the channel you have to hook the cam switch up on your GR 16.
But if you already spent all the money for a Graupner MC32, digging into the transmitters menu and learning its functions would not be so bad
But basically I would check it like this:
1) Check if a signal comes out when hitting the switch of the transmitter by checking the "Servoanzeige"
2) Check if a signal comes out on the receiver --> Do this with an oscilloscope, and if you don't have one, just use a servo and see if it moves when switching the button
3) If 1 and 2 are done successfully and the cam switch still doesn't work, use a channel of the transmitter, where a stick is connected to. Move the stick all the way and check if the switch reacts than..
4) If all this doesn't work, take the cam switch, flush it down the toilete, and be happy with one camera