Position V1 = Clean Channel tube located next to guitar input jack.
Position V2 = Overdrive channel and it runs back through V1 to overdrive tube.
Position V3 = Phase Inverter which collects the tone from V1 and V2 and channels the tone signal into the power tubes.
V3 is located next to EL84 power tubes.
Position V1 forms @ 85% of your tone and V3 the other 15%.