Hello ljos
The master/slave mode is used when you want multiple units to do the same thing.
If you want to control unit 1 from unit 2 without unit 3 to do anything you can use show control.
Lets say you want to control unit 1 from unit 2.
You will have to create a task in unit 2 to send a message to unit 1.
An OSC message should work for this.
Then in unit 2 you should configure a playback to start when the message from unit 1 is received.
Please note that you can select which messages unit 1 should respond to, using the trigger value.
Using different messages for unit 1 and 3, you can control them separately from one unit.
Best regards,
Guido