Re: Getting a Gibson Sg-3
Welcome aboard. SG's are great guitars, good choice.
Some of us here think a single coil works best in the middle of an HHH guitar, as HB's sound very different in that slot for whatever reason (nothing like they sound in the bridge or neck; which has something to do with how they line up with the string nodes, and possibly magnetic field issues). There are a number of companies today that make P-90's that hit a HB slot. I did that with my SG Custom, put in a C5/P-90/'59N.
Do you have experience with the JazzN? It's sometimes considered a rather bland & timid PU. Some of us prefer a '59N or PGN (PAF's) in the neck as they have more color & character to them, and balance better with a hot bridge PU.
Are you doing the soldering yourself?