A second push/pull would allow you to have the middle by itself as well. For example:
- Tone push/pull would select if the middle is solo or combined with the other pickups (when on).
- Volume push/pull turns the middle on/off.
So, with the Tone push/pull down, pulling the Vol up would add the middle to whatever bridge/neck combination is selected by the toggle switch. With the Vol down you just get the bridge/neck toggle output.
With the Tone push/pull up, pulling the Vol up would give you the middle by itself, and Vol down returns to just the bridge/neck toggle output. Quick sketch:
Attachment 43506
There are lots of different ways of doing the 3 pup setup, it just depends what controls you want and how many holes you’re willing to put in the body.