Thank you,
The RC600 User Manual says:
"Transmitting Control Change messages:
If you select a Control Change as the “ASSIGN” (p. 18) target, you can transmit MIDI control change messages by operating the buttons and knobs on the RC-600, or by using an external footswitch or expression pedal (connected to the CTL 1, 2/EXP 1 jack or the CTL 3, 4/EXP 2 jack)."
I think I just need to know what channel ST3 is listening on then. Or does the Listen/Learn function in the Assign Action page listen on all channels?