I am glad the OP started this thread. It may be one of the biggest, most well-circulated sexual myths out there IMHO. You hear it all the time.
In my experience, my wife loves the shallow stuff - say the first 3 inches or so. But she does not have the big O from it. The same can be said for 5-6 inch depth. Once turn on, the deep penetration, about 7-8 inches is what takes her over the edge into big O land.
She describes a very sensitive spot, deep inside that really responds to pressure. It gives here those fully body O's. I presume this the anterior fornix. Whatever it is, it is much deeper than 3 inches.