So adding ESS assistants to an existing 3-phase Multi setup, the recommended way would be connecting only a MK3 to the three Multis
and edit using VE configure tools? When you write “outage” you mean it will shut down when it is missing the GX I assume?
But can be recovered and editing can be finalised after that?
There is no recommended way, you need to choose based on risk if it goes wrong and the ability to maintain remote access.
If something goes really wrong and you need to recover, a mk3 is a necessity, this should always be considered in planning.
Loading ess can be done either way.
Inverters will power off when the configuration is applied. Or if the bms connection times out.
I’m glad you’re doing the research and asking the questions.
It avoids problems 
So if I disconnect a working existing 3 phase system that has BMS master and instead connect it to an MK3,
it should not shut down as it would if one just disconnects the BMS/GX?
The MK3 should stay working even if it is configured with a BMS as master on the GX that it is then disconnected from?
Or do we need to remove the BMS master and reset the Multis before connecting it to VE Configure for sending configs individually to each Multi?
When you disconnect the GX, the system loses access to the BMS and DVCC stops so the system goes into passthrough.
There is a timeout that applies, it’s a number of minutes iirc, before the system will fault with BMS lost.
Don’t mess with BMS settings, rather install a bypass/changeover switch so critical loads can run off mains for maintenance, if one isn’t there already.
This is maintenance where there will be downtime, it can’t be avoided, only minimised.
The system is going to power cycle for the config changes regardless. Chances are, on an older system, you might want to do firmware as well.
You’v been truly kind and helpful. Thanks very much!
The switchover is already in place. Rehearsing for the next phase 