Eldar said:
If you want to do manual control of up to 5 flash groups and they are meters apart, it does take a bit more time. But the benefits of having the built in radios and avoid the PocketWizards outweighs the downside. I may just skip the entire manual ambition and try to make it work with TTL ... but I´m a control freak when it comes to setting up my cameras, so all this automation is a bit against my nature ...
Don't skip M mode, ETTL is an exercise in frustration when you want complete control and consistency, ETTL is good, but it's strength is differing flash to subject distances, not rim lights, hair lights etc.
Just do what has been suggested, hold the ST-E3-RT in one hand and the Sekonic in the other, use the Sekonic in Cordless Flash Mode and fire the 600's via the ST-E3-RT or a Master 600 in your hand, take the reading and adjust the power levels, then repeat, when you are all set up put your ST-E3-RT or Master 600 back on camera and you are done. Full wireless control, no additional stuff in the way.
http://www.canonrumors.com/forum/index.php?topic=19412.msg366450#msg366450