The YN-E3-RT and ST-E3-RT both work the same, the information is not transferred to the remote flashes until you fire the transmitter, you do not need to "pre-fire" the transmitter when used on third party cameras, well my one doesn't! The updated information gets transmitted when you do the shutter sequence.
Also, you can use Group Mode with the YN-E3-RT on third party cameras if you put all the groups in M, this gives you the capability to fire five different groups instead of the three you get in M Mode, but it won't do second curtain sync in Group Mode whereas, as you said, it will in M Mode with the three groups.