The yongnuo 500-something one that I use is basically a replica of the Canon 500-something series.... it works the same for $60. Apparently it's firing can be unpredictable (so I've heard?..), but I've literally never had a single problem.Even at the $370 price of a refurbished 600RT, you can get 5 of these along with Yongnuo remote transceivers and sync cables.
600EX RT is the best speedlite for 5D3. I have two of them, 430EX II and used to have 580EX II before. 600 is easy to use and it just works. Flash head must turn 180 degrees both sides, which 430 can't do -> 580/600 if you will buy canon.