Magic Lantern has an advanced bracketing feature that allows you to set 2-9 frames, at 0-5EV intervals. The bracketed set is taken with a single shutter button press. There's also a mode where it can automatically evaluate the scene to set the number of shots needed to cover the dynamic range.
There's also the dual-ISO mode which will let you basically do HDR in a single exposure by using two different ISO settings on alternate lines of the image which can be software interpolated to fuse the two different exposures.
However, the 5DMkIII is not one of the cameras supported in the latest stable release of ML, so you'll have to make a judgment call on whether you want to risk a 5DMkIII to a nightly build.