I found one for how to make the 3 button switch as well, but the schematics on the message board are not quite helpful.
The thing to keep in mind is to use a TRS jack and connect one button to each combination:
T + R
T + S
R + S
You'll probably want to test it out before you mount it to any box just so you know which button will be doing what to the Mod / Time Factor.
