Hello,
The problem seems to be with Alt-mode technology.
The Dell D6000 relies on two USB graphics technologies to support up to three external displays.
DisplayLink and USB-C DisplayPort Alternate Mode (Alt-mode in short), an optional USB-C port feature that requires connecting the docking station directly to a compatible host computer's USB-C port using the original USB-C cable provided with the docking station.
DP Alt-Mode is used with HDMI if the other two ports are occupied. DisplayLink is used with DisplayPorts if using 3 external monitors. If you use just two external monitors DisplayLink will handle them regardless of what ports they are connected to - this is why you would most likely see HDMI working if one monitor connected to DisplayPort is disconnected from the dock.
If three external monitors are connected DisplayLink will only handle the DisplayPorts and HDMI will not be seen by DisplayLink driver as it uses different technology than DisplayLink.
For Alt-mode to work you have to have Dell D6000 connected via USB-C and your computer has to support DP Alt-Mode. If your computer does not support this technology - the HDMI port (when using all video ports), will not work.
If you are connected via USB-C and you are certain that the computer does support DP Alt-Mode then it is best to consult Dell directly.
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