Yongnuo ST-E2 Transmitter Sync Issues
Yep, Think I read somewhere that the 5DIII also does not work with Yongnuo's version of the ST-E2. So you may be in trouble. It's one of the reasons that I have reservations about Yn's 622C RF TTL triggers, because they have no way of updating their firmware, unlike their competitors. Your issue shows how critical this can be.