The more room you give yourself, the better. Depending on the scene, 3 exposures might not be enough. Some scenes need many more than that. Make sure you are shooting in raw and in manual. The histogram is your friend, along with the high light alert. If you have a lot of time, with a still subject, you begin by exposing the darkest exposure that the highlight alert just shows up with white flashing. You then begin to systematically increase your exposure, using the shutter, not the aperture, by 1-2 stops each time. After you take the shot, look at the histogram and when the left side of the histogram begins to go flat (i.e. there are no more vertical lines), you have captured most of the dynamic range of the scene. This could be any number of exposures. I have done this and have been very pleased with the results. If your title is correct, you have a 5D mk III and that allows 7 frame AEB which I would recommend if you don't want to take the time for the above procedure as I said: the more range you give yourself, the better off you will be. To give you some insight though, before I knew of the above technique, I used the AEB on my 7D with +,- 2 stops for this picture in Sandusky, OH.