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| p.1 #3 · Need Help with 600EX-RT wireless in manual mode |
Just to clarify your question, are you trying to use three flashes -- one on camera and two off camera -- at the same time? And you want to use two different cameras to control the two off-camera flashes?
As you found, the Master is always Group A. You can also set a Slave to Group A and the other to Group B, or you can set one Slave to Group B and the other to Group C. (And with the radio mode of the 600EX RT, you also have Groups D, and E if/when you get more of them, but unless you have a 2012-model camera, the latter Groups may not be available.)
In ETTL mode, Group C operates differently than A and B, but in Manual mode all three work the same way, with you setting the levels of each. Manual mode is definitely the way to go.