There is a difference between true single coil pickups and single coil sized pickups. Humbuckers are always two coils, wound in opposite directions so that the wall current of one cancels out the other (generally referred to as 60 cycle hum, which is audible from most true single coils).
In an HSH setup, the H's refer to the size of side by side coil layout pickups, like a PAF or EVO. The S means single coil sized pickup, not necessarily a true single coil pickup. You could use anything you want that will fit in any of the available holes (well, that's what she said anyway...) be it a stacked humbucker or a true single coil.
So, you could even use a single coil/humbucker/single coil in an HSH config, if you chose the right pickups, like Duncan Phat Cats (humbucker sized P-90 style singles) for the H spaces and any of the hundreds of stacked humbuckers for the S slots.
If all of this is over kill just ignore it
jim