To keep it simple connect three log volume control potentiometers to an audio preamp IC output and wire the sliders of the pots to the hot pins of the output connectors. Use low resistance, say 2 kOhm potentiometers and be sure your preamp can feed 600 Ohm well. Use stereo pots for 2 channel stereo configuration.
If you need level setting switches or long cable feed (5 meters or more) ability this is too simple.