Dell D6000 relies on two USB graphics technologies to support up to three external displays.
DisplayLink and USB-C DisplayPort Alternate Mode, 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 DisplayPorts are occupied and 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 (DP+HDMI or DP+DP).
If three external monitors are connected DisplayLink will only handle the monitors connected to DisplayPorts and HDMI will not be seen by DisplayLink driver as it will use different technology than DisplayLink.
This is why you only see two monitors listed in the DisplayLink Manager - it will alsways be two monitors with Dell D6000. The 3rd monitor can be controlled by the native OS display settings - including rotation.
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