well, with the appropriate schematic i could do a pinout diagram... I only have the documentation for the Triumph and Ultra Plus footswitches on-hand, though.
With said diagram, one could make the cable fairly easily.
EDIT: I'm fairly sure that the 3120 uses the same switch as the XXX and JSX.
From the solder side of the jack, from top right:
Pin 1. +17VDC supply to pedal
Pin 2. not used
Pin 3. Ground this pin to detect footswitch
Pin 4. Ground/Common
Pin 5. Clean/Bypass
Pin 6. Ultra/Crunch
Pin 7. Effects
so, you wouldn't use the wires from Pin 1 or 2, but I'd need more time to work out how else you'd wire things together. Maybe someone who does this more than I do would know more.
I think you'd solder pin 3 to pin 4, and then have 3 1/4 jacks from 5, 6, and 7, with the grounds all going to pin 4? I'm not sure entirely, someone should fact-check me.