Epiphone Black Beauty 3, Middle P90?

I've tried the 3 HB and now a 3 P90 guitar and haven't been happy with either. I like having the neck-bridge combo and putting a middle on a separate volume or blend kob to add when you want it is a good idea.
I'm converting my 3 P90 middle pickup to a middle Strat pickup which is fitting since it's a Strat guitar anyway.
Changing the middle pickup to a P90 on your 3 HB guitar may leave visible gaps, but using a HB size P90/P94 pickup would work great. The SD Phat Cat is a fantastic pickup.

 

I have a three pickup SG and put vintage-voiced pickups in the neck and bridge and a much hotter one in the middle.

 

I just got done putting a TVJ T-90 into a 3-pickup Gretsch just last night actually if you're interested in reading about that:
https://www.gretsch-talk.com/threads/anyone-try-a-p-90-in-the-middle-im-about-too.208056/
One of the things that motivated the build was my desire to be able to run the mid by itself, which I got a wiring diagram for. It has a typical B+N 3-way and had another on-off that brought the mid in, always with one or both of the others. I converted that on-off to another 3-way that gives me B+N only, standard operation; M on with the normal B+N switching; and M only.
I'm still getting her set up after being disassembled for so long, but so far I'm really liking that M-only option, especially with that T-90. Adding it to one of the HiLoTrons gives it fatness underneath the chime, so defo something I'll be exploring tonight after logging off.
P-90's generally use the same pot and cap values that buckers do so you're good there. I actually used a push-push pot on the tone to allow me to use 0.047 or 0.022 caps, sort of a mini varitone, and the 90 actually seems to like the 0.047 more, though the difference is subtle.

 

cutting a new pick-guard isn't that much work

 

Strumming without hitting those extra tone knobs might be, though.

 

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I'd suggest converting your triple humbucker guitar:

neck: Gretsch/TVJones Filtertron kind of pickup
middle: P90 in a humbucker case
bridge: Split humbucker

Regular 3-way switch for neck and bridge combos

Master volume, master tone

A 3-way on/on/on toggle switch for the bridge humbucker to get Parallel/Single screw pole bobbin/Series.
If you play with pickup height and screw pole heights you can get Strat #2 Quack out of a bridge pickup set up this way in parallel (I have a Tele Esquire-H on a 4-way switch that does this). Mounted in the third pot hole.

On a stacked, concentric set of pots for the last standard control hole in the top:

Base ring is wired to the P90 as a blender pot so you can range from full P90 back to the base HH guitar output, or anywhere between.
(Brent Mason's gray Tele is wired something like that, but with a blade humbucker in the middle).

Tip knob of the pair is wired with a Reverend style master Bass-Cut circuit. This circuit works great for distortion/heavy drive.


... You'll have all the tones.

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Cutting a new guard isn't hard for me, but the OP has a 3 HB LP - no big pickguard to work with.