Ok, so hopefully someone here can help or give insight.
I'm encountering issues with this set-up:
2 600EX RT's set up on stands, and 1 600EX RT on camera.
When the 2 off camera flashes are behind the subject, the on camera flash underexposes when in a bounce position (works ok when straight on). Camera is in M mode. This was tested with a low, white ceiling.
I've tried this in both full ETTL on all flashes (in group mode), as well as setting the off camera flashes on M and dialing them way down and leaving the on camera flash on ETTL. I'm guessing the camera meter is seeing the bright flashes behind the subject as a well exposed scene and thus will not give a lot of fill? I've tried both spot and matrix metering with similar results.
The idea is to create rim/side light, while the subject is correctly exposed, and turning off and on various flashes as needed. This is all in Group mode. I want to use this type of lighting for wedding receptions.
Another issue I'm encountering, is when in group mode, if I turn off the OCF's (from the flash menu), the on camera flash won't fire, even if it's set to fire. I will have to take the flash out of group for it to fire.
Hope this all makes sense!