I believe for PS you actually need to have different exposures. Btw, you're not limited to 3 exposured, you could 7 or even more if you were inclined. Also, originally the idea of HDR was not a correct with an over- and under-exposed image, but rather seperate images exposed correctly for various aspects of an image, then combined to get all parts of the image exposed correctly. Semantic difference, I know...
^^Right, i've done 20 exposure HDR's before, it just that 3 seems to do the trick 90% of the time.
Now you can make a tone-mapped image from a RAW file just with exposure compensation from CameraRAW, but that exposure compensation is nothing like the real deal.