Thought I'd drop these nuggets for anyone looking to wire (3) P90s into their guitars.
First, the middle pickup must be reverse-wound, meaning you would wind the pickup clockwise. The neck and bridge pickup are winded in counterclockwise.
Polarities of the magnets for the neck and bridge can either be North or south, but they must be the same polarity identical to each other. The middle pickup will be the opposite polarity from the neck and bridge.
So your pickup orientation can look like this:
One thing most of you notice when wiring more than one pickup, that when you switch to a neck or bridge pickup, the sound is loud and clear, but when switched to both pickups, one of the pickups volume reduces drastically. You will still get an interesting tone if you like it, but if you want full throttled pickups with independent volume control without one pickup drowning out the other, you would need to solder your volume pots in reverse. Yes, for all 3 pots.
Instead of soldering a pickup to Lug 3 for the input, solder it to Lug 2 as the input which is the wiper. When you solder Lug 3 as the output to the jack, the signal passes to your amp without any shorts and thus keeping the volume of the pickups at most desirable levels, not to mention full control from the potentiometer.
Here is my crazy diagram, notice the Tone pot goes straight to output jack. Also, I added a dirt circuit for some grime consisting of (2) diodes and mini toggle switch (SPDT or DPDT will do; just need to make an on/off switch.).
First diagram shows to wire the hot, the second shows how to ground it all.
love P90s. Thank you for your complete outline on this