When you select an input signal, you want the other signals to either disconnect or shut off. Old-fashioned sound systems had interconnected mechanical switches. Pressing a button caused the other buttons to pop up. You need to do the same thing electronically.
There are logic gates to perform these tasks. A 3-input AND (or NAND) gate comes to mind. Add invert-gates before one or 2 inputs as need (example, so that the output goes High only when input #1 is High, and the other two are Low).
Another trick is pullup/pulldown resistors. These create a default condition at a terminal. To override the condition, apply a signal that comes from a lower impedance. Or sometimes through a diode (oriented in one direction or the other) if you only want for a certain polarity signal to influence the condition.