First of all the ST-E3 is a Speedlite Transmitter, so nothing can be received with it. Second, the 600EX-RT (or any Canon Speedlite for that matter) cannot trigger the camera to fire (out of the box, at least).
Wow - two wrongs there, and unfortunately, they don't make a right.
The 5DIII and 1D X can be triggered by a 600EX-RT mounted on the hotshoe, from another 600EX-RT connected as a slave unit. Pre-2012 camera bodies need the Release Cable SR-N3 to connect the 600EX-RT to the remote terminal for triggering.
An ST-E3 on-camera can also trigger a 5DIII or 1D X shutter from a 600EX-RT connected as a slave (meaning, yes, the ST-E3 can receive radio signals, too). Again, the -N3 cable is needed for older cameras to connect the ST-E3 to the remote terminal.
Is it possible for me to shoot with my 5D II with an 600 EX RT, and to trigger at the same time the 5D III which stands on a tripod at the same time? If so, does anyone know how to?
You can't trigger the 5DIII by firing the 600EX-RT on the 5DII, though. You can manually activate the release from the 600EX RT to trigger the 5DIII with the ST-E3 on it. Check the "Remote Release from a Slave Unit" section of the manuals.
With additional hardware, you could trigger both cameras simultaneously - in fact, you can link up to 16 cameras together for simultaneous shooting, if you have enough ST-E3s/600EX-RTs).
If you can get a hold of a Release Cable SR-N3, what you could do would be to connect that to the 5DII's remote terminal, then set up Linked Shot mode, with the roadside camera as Slave and the distant camera as Master (either camera would work for either, AFAIK). Then, the pressing the shutter release on the Master would trigger both cameras to fire at the same time.
From looking online, it appears that Adorama has the Release Cable SR-N3 in stock.
Thank you Neuro and Jerry.
I won't have time to get the SR-NR before the happening, but I will get one finally. I thought that the LC-5 might not work due to the angle. The only option left to me as I see it is to get this Phottix trigger:
"The Phottix Aion Wireless Timer and Shutter Release takes wireless triggering to the next level. The revolutionary wireless/wired timer and shutter release offers photographer an amazing number of triggering options, including auto-bracketing for HDR.
What does the Phottix Aion offer?
•Timer and Long Exposure Functions
•Shutter Release Functions
•Wired or wireless operations
•2.4 GHz frequency
•Self, interval and long exposure timers
•Number of frames setting
•Auto-bracketed timer setting for HDR photos
Two-stage shutter button for AF and shutter release
Instant, continuous 5 shot, 2 second delay and bulb shutter release mode.
Phottix Aion sets for Canon, Nikon, Sony and Olympus are available.
•Range: 60 meter
•Storage Temperature: -10 C to 60 C
•Standby Timer: 48 Hours
•Transmitting power: ≤-10dBm
•Working voltage: 3.0 V
•Size: 130.4 mm X 46.2 mm X 22 mm
•Receiving sensitivity: -90 dBm
•Working voltage: 3.0 V
•Size: 85 mm X 37.4 mm X 30.9 mm
•Weight: 37.5 g"
I hope it works with the mk III, and I will call the store tomorrow.
Then I can stand with my mk II and my 70-200 and shoot him as he is approching the bend, and as he is speeding away again, and then trigger the other camera with the phottix with my left hand:)