This is more confusing than I thought it would be.
So if I get 2 600 flashes, I can use one to trigger the other? Even if they're both off camera???
You'd use 1 of the 600's on camera (or the ST-E3), with the second one off camera. The one on camera talks via RF to control the one off camera. Canon put the IR Master function into a few cameras (60d/7d/??) and there for you could trigger an off camera flash, but that feature wasn't part of the 5dmk3.
If you're looking for a stop gap to have a single flash off camera, get one of the really long eTTL cables like http://ocfgear.com/
sells - yes, it's wired but you have complete control of the flash from your camera body - AND it's good to have a wired backup just in case. Syl has a number of books that people like specifically on Canon flashes, and a good amount of info on his blog http://pixsylated.com/blog/
The reason we're saying to not purchase the PocketWizards is that if you went the cheapest method possible - a MiniTT1 and a FlexTT5, to add a second 600ex-RT would require yet another FlexTT5. Where as if you purchased a ST-E3-RT or an additional 600ex-RT, you would save the cost of the PocketWizards, plus the overhead of added, unnecessary gear.