Problem persists with Ubuntu 17.10 / DRM 2.49.0 / 4.11.0-041100rc8-lowlatency,
LLVM 4.0.0

Tested all resolution options for all three of the monitors, and has been able
to output either on HDMI and VGA or HDMI and LVDS (internal). I have no idea
what else could I test (or which code could I look at). Would setting drm.debug
flag  to 0x4 in GRUB provide some more relevant data?  

Also, I have found this forum thread about the same issue, which dates from
2013: https://ubuntuforums.org/showthread.php?t=2194601