OK, let me put it this way:
Name a tone that you require that was done/can only be done with a middle HB.
I dont doubt that there are more sounds in a 3HB guitar than a 2HB. It has to have more!!
But theres a big difference between screwing around with pickup combinations and going "Ooooh, the south pole of the middle pickup in series, out of phase with the north pole of the neck pickup sounds cool!", which a 3HB guitar is a playground for, and having specific tones in your mind that you want to achieve and going for a 3HB guitar to do it.
I think you can get a very good approximation of all of what you want (provided you have the right amp for the job: Engl invader or something equally versatile) with HH or HSH.
I think that putting a HB in the middle, with the different vibration profile of the string there, will certainly give new and fun sounds, but you may well find yourself moving away from the sounds you want: nearly all of which are neck/bridge HBs or singles: calling, therefore, IMO, for faily powerfull pickups in HH with coil splits, and paralel wiring for softer tones.