Re: 3 humbucker configuration - help!
Every 3-humbucker guitar I have, or have had, it seemed the intent of the middle pickup was to soften, mellow out and warm up the treble pickup; at least that was the effect I heard. I haven't come across any 3-humbucker guitar that was wired to allow the middle pickup by itself, yet.
But I will say, if you do a typical 3-HB wiring, you will want the middle pickup to be weaker than the treble pickup; or else you'll want to lower the middle pickup toward the body more than the others. Because if it's a hot middle pickup, it will sound like loud mud when switched in with the treble pickup.