If distribution board 3 only has “office lights and sockets” you should be fine taking the 3 sockets from there, 1 and 2 may be slightly less suitable depending on how heavy the loads are on them, but 3 is close enough (electrically) and unlikely to have a voltage drop of any significance.
When installing those sockets you should consider hiding them away in an enclosure.Tthis is not only tidier and less likely to get unplugged, it is also safer and less likely to contravene any regulations (or bad practice) about having sockets from different phases in close proximity due to the potential 415v across any 2 sockets in a fault condition.
I have installed a fair few 3phase systems and I usually do something like this (excuse the 4way extn lead, this pic is from the test bench not installed on site)
Although I have now progressed to using 2 smaller enclosures to separate the high voltage stuff from the monitoring electronics as it is more flexible to locate than one big box and the Pi runs cooler without the heat from the 4 PSUs
This is a later monitor box, the 4 sockets (2single + 1double for the 5vdc too) are now put in a separate box by the sparkies doing the electrical install.
Also if you look closely you can see in both pics the 3 emonTx’s are all connected via USB (using serial adapters) rather than using RF