I played around with the HDR function of the 5D3 this past weekend while we running around Traverse City (in northern Michigan). The 5D3 will take three successive exposures and then combine them in camera to produce a jpeg image. You can specify exposure bracketing of +/- 1, 2 or 3 EV (there is an auto mode, too). The three exposures are also saved separately as RAW files (assuming you are normally saving in this format). Canon's DPP software for RAW processing also has an HDR utility for combining these image files, allowing for finer tweaking of the image before creating the jpeg.
Here is an image with the in-camera processing using the "natural" setting:
Here is the same image using DPP's HDR utility with some fine tuning:
And here is a single exposure photo of the scene with my typical adjustments for processing the RAW file. The framing is slightly different, as these were handheld:
The above was probably the best HDR results I got. In most of the other shots, the foliage ended up looking off due to movement caused by the wind.
Here is an example. The first is using the DPP HDR utility, and the second is a single exposure. The in-camera image didn't look any better than the DPP created image.