I spotted this switch usage some years ago, I think in a madbean design.
Basically this wiring converts a DPDT on/on/on toggle switch to a SP3T switch.
It's an easy, inexpensive way to mod a pedal, say to add a 3rd switching option to a SPDT design.
Been using it ever since to select 3 different diode clipping options, or 3 different caps values.
I used it most recently in the ProCo Solo and the Madbean Turnabout to select the 3 independent clipping options.
I've also used it to select 3 different speeds in a Chorus and in a Phaser.
Smaller and cheaper than a 3T rotary :0)
The only catch is there are 2 different types of on/on/on switches (ask me how i know), each having different lugs shorted in the center position. Just need to check which you have, and wire it up as noted below. (I labeled them Type "A" and Type "B" which is not official, just my way of identifying each type)
1978 Gibson Les Paul Standard, Cherry Sunburst