I came across this website:
https://en.wikipedia.org/wiki/Audio_crossover
This is one of the few references to "three-way" I came across my investigation as to why a subwoofer would have three voice coils:
"Crossovers are often described as "two-way" or "three-way""
So my latest hypothesis is that this cambridge subwoofer uses different wattage levels to reach different frequency ranges.