ThunderBolt 3 dock triple display

Question

I have an Intel® NUC Kit NUC8i7BEH with an Elgato Thunderbolt 3 Mini Dock and I can't make it work with three simultaneous displays. The NUC says it supports three displays and the dock says that it supports two at the same time using a DisplayPort and an HDMI port.

I can only use two displays at the same time, the integrated HDMI of the NUC and only one of the ports of the dock, when I unplug one display from the dock the other becomes active instantaneously.

I am using Ubuntu 18.04.3 LTS with Linux 5.0.0-32-generic and the latests Intel BIOS.

The ThunderBolt device is active and in insecure/legacy mode on the BIOS (Dock's USB port works OK):

> sudo boltctl list
 ● Elgato Systems Thunderbolt 3 Mini Dock
   ├─ type:          peripheral
   ├─ name:          Thunderbolt 3 Mini Dock
   ├─ vendor:        Elgato Systems
   ├─ uuid:          99999999-9999-9999-ffff-ffffffffffff
   ├─ status:        authorized
   │  ├─ domain:     domain0
   │  └─ authflags:  none
   ├─ authorized:    Mon 01 Jan 2019 02:47:00 AM UTC
   ├─ connected:     Mon 01 Jan 2019 02:47:00 AM UTC
   └─ stored:        Mon 01 Jan 2019 07:34:26 PM UTC
      ├─ policy:     manual
      └─ key:        no

Xrandr has the same output with or without the dock plugged and output DP-2 is always the active display that is connected to the dock, even if it is using the HDMI port:

> xrandr
Screen 0: minimum 320 x 200, current 3840 x 1200, maximum 8192 x 8192
DP-1 connected primary 1920x1200+0+0 (normal left inverted right x axis y axis) 582mm x 364mm
   1920x1200     59.95*+
   1600x1200     60.00  
   1280x1024     75.02    60.02  
   1152x864      75.00  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   640x480       75.00    59.94  
   720x400       70.08  
DP-2 connected 1920x1080+1920+0 (normal left inverted right x axis y axis) 509mm x 286mm
   1920x1080     60.00*+
   1600x900      60.00  
   1280x1024     75.02    60.02  
   1152x864      75.00  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   640x480       75.00    59.94  
   720x400       70.08  
HDMI-1 disconnected (normal left inverted right x axis y axis)

Please any hint, advice is welcomed, I searched everywhere but I can't make it work.

Thanks!!!

Answer 1

Here is a support page for someone else with the same exact problem. Apparently this problem might be a limitation of the NUC8 hardware.

"System manufacturers have discretion when designing their systems as to how many DisplayPort video lanes are routed to each Thunderbolt 3 port. If there are two DisplayPort lanes routed to the port, then two displays are possible via a Thunderbolt 3 device. If only one DisplayPort lane is routed to the port, then only one display is possible via a Thunderbolt 3 device."

Edit: added quote