The Phones output (by default, if you don't have any PFL or AFL buttons pressed) is just going to be a copy of the Main Mix - but its level is independent of the Master Fader. So what you're hearing out of the mains is also going to be coming out of the phones output, but the position at which you've got the master fader doesn't affect how loud your headphone mix is.
For what you're trying to do, you'd want to use the Monitor mix as opposed to the headphones mix. This way, you can send just channel 3/4 to the main output and both channel 1 and 3/4 to the monitor output.
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