|What's this good for
In lieu of a traditional pickup selector switch, many guitars have individual On/Off switches for each pickup. That can potentially allow greater control and newer options (e.g., all three pickups turned on at same time), but there's a trade off in that this is almost never as fast as employing a single switch to select combinations of pickups.
How it's done
Of course, you will need an On/Off/On DPDT switch for this mod. And it's all wired together as shown. As usual, the colors of the wires depend on those of pickup manufacturer. Consult the paperwork that came with your guitar, the manufacturer's website, or see this page. In this diagram, the red wire is the output from the pickup.
An alternate approach is to try the series/tapped/parallel mod. It gives one more tone, but requires a pickup selector switch, but you probably have one on your guitar already, so consider that instead.
If you want to change the phase of a coil in the humbucker, place a phase switch between the pickup and this switch.