HDR = High Dynamic Range, also HDRI = High Dynamic Range Imaging. That's when several identical or near-identical images are taken at different exposures. Then, we use HDRI software to generate HDRI image. Usage eg. interior of flat with backlit window. As the camera is unable to record details cos beyond brightness range of camera. So the net result = u can see the details of the window and not just white space. Therefore, use HDRI to solve this overexposed-cum-underexposed problem in an image.
Not easy to take different exposures cos if camera moves even slightly, alignment problems will occur resulting in ghosting in final image.
But if u want to avoid this problem by taking just one image and then using photoshop to vary exposures, and afterwards make a copy of each exposure and then merge the differently exposed images into one, that's NOT TRUE HDRI.
Soon, we'll be seeing HDR photo sensors in the market..
Your final HDR output desired is brighter that any of the three brackets you took. You will get a noisy processed shot since the program will end up pull more details from what ever underexposed shots bracketted.