Good day,
I'd kindly like to know if its possible to backup L3 with Multi or Quattro and leave L1, L2 without Quattro as shown on the picture. Backfeed is allowed on 3~ 400V 50Hz grid. 
asked
3 Phase grid with 1 Phase ESS
Yes, would work. However whe the grid fails, the PV will shutdown, and your will only have 1 fase with power from the batteries.
The PV won't support the loads since its on the grid side, so you only rely on the batteries.
You might consider rewiring some panels to a separate MPPT at the DC bus, so if sun is available it will extend your runtime from the batteries.
Only issue is an outage on not all 3 phases. If only the phase with the MP fails, you might experience some strange issues, since the PE and Neutral are connected by default with the ground relay, and single phase loads on L1/L2 will use the Neutral...
Don't have specific experience, but I think you need to apply an external automatic breaker which disconnects L1 and L2 when the Multiplus is providing power
In Estonia where I live we have several times a year power failure and since people are not rich here, I install such system to my customers too.
Sometimes including singel phase sell back.
Yes it is possible to use it as a UPS on 1phase but make sure you take the right anti-islanding precautions. If the anti-islanding relais of the multiplus II is allowed in your local grid regulations, then make sure there is no 3phase device (like the borehole pump on your image) feeded thru the multiplus !
As your 3phase solar is connected on the grid side, it would not be able to produce power in case of a grid failure. therefore the UPS will fully depend on your battery capacity. if you expect some grid failures during the day it is wise to add some solar on a dc coupled MPPT charge controller. That way your battery will last longer.
