Took my Fuji XE2 and YN-E3 along on a wedding job today.
Had a chance to test the ability to use Canon 5DIII with 600EX-RT on camera and two 600EX-RTs on stands. The stand lights were in Group B and C in Gr Mode ETTL on the Canon.
Then, with YN-E3 on XE2, in Gr mode with A turned off and B and C in M mode. B set to 1/4 power and C set to 1/8 power. I pressed the test button and then shot with the XE2s fired as expected and gave me the proper exposure, once I had the camera set for the lights. Then picked up the 5DIII and woke up the on-camera 600EX-RT and it changed the lights to ETTL for proper exposure.
I had the camera settings enough different to ensure that the lights were changing power settings.
This is a very neat capability.
Just to confirm that everything was working as I expected, I replicated the situation this morning. First Canon with ST-E3-RT in Gr mode and ETTL. Two 600s, one in group B, one in group C. Fired and got expected exposure.
YN-E3 on Fuji XE2 in Legacy and Gr M mode with Group B at 1/4 and Group C at 1/8. Without pressing the test button, the two speedlites stay in ETTL mode and fire at the power setting established when controlled by the ST-E3. Press the test button on the YN-E3 the speedlites change to M mode and show the power setting.
Fire again with 5DIII and ST-E3, the speedlites switch to ETTL mode before firing.