I have been using IoShutterPro on a 60D for shutter control. It as an ap for Ipads or pods that has a dongle that connects to the camera to fire the shutter. I know they have a 5DII interface, I would assume it works on the 5DIII but you should check first.
It allows you to program shutter time for long exposures, has very flexible time lapse modes, trigger by sound, trigger at a particular time, and lots of weird combinations of functions. I use it a lot for multi-minute exposures in astrophotography and have shot some cool time lapse sequences with it....