That terminal on the SRT-E3-RT is for connecting it to a camera's shutter control terminal - in the same way that you would connect any remote release (e.g. RS 80N3) to the camera. Then you trigger the camera from another flash that is talking to the SRT-E3-RT - or so it reads; I have not tired it.
I initially thought the same thing - that I could do what you are saying to integrate the Canon flash into another system, but the SRT-E3-RT user manual says no - it is a shutter release connection terminal.
What might work is connecting brandx receiver to the PC Terminal on a 600EX-RT set as Master that would in turn trigger Slaves 600s. I have not tried that, but might be worth a go...
Steve
EOS 5D MKIII
