You can reach out to a Dingwall dealer or Dingwall themselves for pricing. It was $200 Canadian last I checked, they give you a third pickup, a new pickguard, and a new 6-way rotary which is built for their 3 pickup configuration. On your combustion it should change the Bridge/Neck Series setting to be Bridge/Middle series and everything else would be the same.
Usually it's because the "sound" of a type of bass often has to do with the pickup placement and wiring. A P-bass is a split-coil closer to the neck in series for boosted low mids. A jazz bass is a bridge and neck pickup wired in parallel to blend between. A musicman is a bridge humbucker usually wired in parallel but sometimes switchable to series.
So when thinking about a 3x dingwall with 3 pickups in a row, you can cop some of those combinations.
Bridge solo, Middle solo, and Neck solo are just pickup sounds at those positions. Not unlike a Jazz with pickups there. But their pickups are all split coils too, so you can go series/parallel inside the pickup. Neck only or middle only in series might get you more like your P bass tone.
Neck and Bridge in parallel is like your Jazz configuration, since that's what the blend sound of a J-bass is like.
Bridge and middle in parallel is closer to the two parallel coils of a Musicman, and series is more like a MM pickup in series (purely based on placement and wiring).
And if you have series/parallel switches on all three pickups you can vary those too.
But so basically:
Bridge/Neck parallel - Jazz bass pickup arrangement
Bridge middle parallel/series - MM pickup arrangement (usually parallel, but some were switchable.
Then the individual pickups might have placements similar to other basses, like Jazz Bridge or neck for bridge or neck, P-bass placement is somewhere in there. Middle might be like the neck on a warwick thumb, etc.
Spend the $90 like I did and get the pickguard that puts both pickups at the rear. Then you will have your middle pickup to rest your thumb.

