Canon’s ST-E3-RT was a disappointment for low-light and event photographers because, unlike it’s predecessor, it lacks the AF assist beam. If you need the AF assist, you’re forced to use a full fledged 600EX-RT, but what if you don’t actually want the master unit to produce any light? Simple. Pop the flash onto your 5d mk III, and use the following steps (also works on a 1D-X.)
To speed up this process in the future, consider adding the “External Speedlight control” menu item to your “My Menu” on the camera.
Don’t forget you’ve made this change, or you’ll make yourself nuts the next time you need the flash to work!
There’s also access to this setting through the menus on the flash itself, but I find this to be the fastest way to do it.