As I understand your request, you wish to add a third pickup between the 2 humbuckers. The third pickup has 2coils (3 wires-2 hot and one ground) that you wish to be able to select.
First, since you now have 3 pickups, I suggest you use a Fender type 5 position switch to select various pickup combinations.
For coil tapping, you can use the same volume pots with pull up DPDT switches for the humbuckers. For the new dual coil pickup, you can add an additional volume pot/switch or use a separate DPDT toggle switch to select coil A, coil B or BOTH. To do this the DPDT switch needs to be a ON-ON-ON style. See the Build Doc for The GPBC SUNN T for a wiring diagram that will permit this switching.