Three layers are common only in high end highest quality construction. The two layers in high quality construction will both be thick sheets (as will the third when present), but the two layers may achieve a combined thickness of only half-that in cheaper construction panel overlaid by plywood subflooring. At the highest end, or in select rooms of the building there might well be three sheeting layers, and such stiff subflooring is necessary to prevent the cracking of large floor tiles of or more on a side, and the structure under such a floor will frequently also have extra "bracing" and "blocking" joist-to-joist intended to spread the weight to have as little sagging on any joist as possible when there is a live load on the floor above.