Hello and thank you for posting!
Thank you as well for the detail you have provided that is much appreciated.
As you mention, we do not support the use of the UD-3900 on the Mac platform (we have not supported the UD-3900 on the Mac platform since the release of Mac OS X 10.9 in 2013 due to performance issues within the operating system) however we will of course do our best to try and help point you in the right direction.
That said, the behavior you describe sounds similar to what has been reported by other’s in DisplayLink’s support forum here --> https://www.displaylink.org/forum/showthread.php?t=67124 and here --> https://www.displaylink.org/forum/showthread.php?t=66858 (DisplayLink is the separate company that makes the primary chip within the dock, and who does the software driver development).
Based on information in those threads, it would appear that uninstalling the current ‘classic’ or ‘legacy’ DisplayLink driver for Mac and then installing the new ‘DisplayLink Manager App’ application has helped others with a similar problem.
DisplayLink provides more information about this new application here --> https://support.displaylink.com/knowledgebase/articles/1932214-displaylink-manager-app-for-macos-introduction-in which also includes specific instructions on how to update to the new application --> https://support.displaylink.com/knowledgebase/articles/1932214-displaylink-manager-app-for-macos-introduction-in#install
** Please note - one of the current limitations with the new DisplayLink Manager App is that ‘Closed-display mode’ (also known as clamshell mode) is not possible. This means that the laptop lid must be open and the laptop’s internal display be active in order to use the dock.
I know you mention that the final result you wish to achieve is to have use of all three displays (the laptop’s internal and the two external connected to the dock) so this limitation will not be relevant in this specific case, however I wanted to make mention for the benefit of any others following this forum thread **
Please let us know if that information helps!