If you are using a phone, iPad, iPod, or computer, start with all of the volumes all the way up.
Next listen with the headphones and turn down the loudest device to match the volume of the other two.
You want all channels transmitting approximately the same volume so that when users switch channels on the headphones, they don’t have to adjust the volume knob.
If you are using a DJ mixer, they go a lot louder, so, you want to be listening in the headphones while you turn it up. Have your own headphones at about 50% volume when you do that. That way, nobody will be complaining about their headphones being too soft when they turn them up higher.