Well, I think you should think harder about the answer before you ask the question. Think about why people would want a humbucker. Anywho, here's my take on at least a part of your question.

It is very rare that you would find a guitar with 3 humbuckers that operate all at the same time, however I'm sure some people do wire their guitars this way. Humbuckers do put out more volume than a single coil at the same set volumes, so for example, someone may want a middle humbucker instead of a single because that person likes the sound of the middle pickup, but may want more chunk out of it.

There would be more tone variation if the pickups were wired for series/parallel, phased, or coil tapped, however if none of these are done then you would have the same amount of tones as any other 3 pickup guitar with the same amount of switches.