I can guess at a couple of reasons that Series instruments with more than two pickups are rare (and, now that you've raised the topic, it's only a matter of time before a 3-pickup one is uncovered somewhere!).
The Series instruments are quite a different beast than all other guitars and even other Alembics. The pickups on the other Alembics are custom-built and custom-wound for a unique voice but are relatively conventional - you could drop the pickups from a Spectrum into a regular guitar without the electronics and it should work.
With the Series instruments, it's a totally different beast. It's a cost-is-no-object implementation of single-coil pickup design. The magnetic aperture is wider for a different tone and the pickups are wound with less wire. With less resistance and inductance, you get flatter frequency response. The output level would be very low though, so each pickup has it's own onboard preamp.
These single coil pickups would still hum on their own. Rather than add a second humbucking coil like most guitars, the Series uses an active hum-cancelling coil. Humbuckers work pretty well, but cause loss of treble since the second pickup coil sees the string in a slightly different place and the mixture of outputs cancels out some of the highs. The dummy coil in the Series instruments doesn't have magnets and doesn't see the string motion at all, just the ambient electromagnetic field which it can cancel out of the other pickup's outputs. Having this work properly requires the pickups and dummy coil have to be carefully built as a matched set, and in this case, a Series instrument has a third preamp for the dummy coil. Only Alembic would add an onboard amplification circuit to amplify the ambient noise so it could remove it fully!
Most Series guitars also have independent onboard Q- or EQ circuits for each pickup.
If you wanted a third pickup, I'm sure it can be done. But it will require yet another preamp/EQ board and more knobs and switches. This is probably the big barrier - cost, weight, and complexity. The dummy coil actually turns out not to be an issue - you can place it anywhere on the guitar that isn't shielded and it should work fine.
The other problem with 3-pickup setups are that raise problems with unusual switching arrangements - they require a lot of switches to unveil all the combinations. Off-the-shelf switches may not do what you want; custom switches could be made but have to be ordered in enormous quantities and would be expensive. Your pickup selector in c) would be a good example of that. It's pretty much like a 5-position blade selector except in the middle position where it's something completely different. A blade switch is a relatively simple double-pole switch, but would require some major hacking to switch the middle pickups for the blends but select a different combination in the middle position.
David Fung
