I'm a big fan of 3 pickup basses. Was searching for threads and pics of examples out there but its slim pickins. Love to see some pics of customs with three separate pickups. Comments on wiring and what you like/dislike also would be interesting.
I have built three triples (pictured below). Two use a 3 way switch on the middle/neck volume with a separate volume for the bridge and master passive bass & treble controls.
The J bass also has a series-parallel switching for the two J pickups and phase reversal on the bridge (push-pulls). It pains me to admit, but it currently does not have series parallel switching within each of the split J pickups.
The Air Force Bass has on/off toggles and volume controls for each pickup along with master bass and treble. I like the VVV with three switch config best so far.
Next on the project list are to swap the bridge pickups in the P and the J. The J will get a double J quad coil (I plan to glue two Dimarzio Area J's together). The P will get an 8 wire quad coil MM in the bridge (probably Nordstrand).
Not sure about controls for the redone P and J yet. Would like to do the 10 option wiring thing on the quads which will probably require more routing and a bigger pickguard (to make room for rotary switch and/or 3 way toggle for each coil).
Am also thinking about a new build with three quad coil MM pickups - each with all 10 combos and maybe with 3 separate jacks plus option to route all 3 to a single jack.
Not sure about preamps. The AFB has a Bart TC-3 (boost only - no tone control). With three pickups you get so much tonal flexibility it almost seems unnecessary (did I really just say that).
Unnecessary complexity and overkill is a design objective for me most of the time, so preamps for all (eventually) with bypass switches of course.