On Friday (10/08/2018) I tried the latest 2018.Q3 drivers. I was able to enable the MGPU eyefinity and setup my 4x3 display. Things appeared to be working well until on Monday (13th) the screens started randomly flicking to black then re-appearing. This appears to be independent for each display and occus every 10-20 seconds for each display. I've tried cleaning the drivers (AMD Clean Utility and DDU) and reinstalling but even with two displays connected and no Eyefinity the same flickering occurs. Also, I can't open the advanced settings page as it just disappears after 1-2 seconds, sometimes (rarely) with a .NET threading exception.
Since that I've run a .NET repair, and clean installed the 2018.Q2.1 driver that I had before, which puts me in the same position as before. That is, everything seems to work just fine, the screens don't blink and Eyefinity doesn't work on more than 6 panels.