@bill555  I have a triple display setup and I can only get Windows to duplicate between two displays as well.  I've never seen Windows duplicate across three displays.  But if that's what you want, then the easiest solution is probably an HDMI splitter.  It will take a single source signal and replicate it out of multiple outputs.  In that case, your system won't know there are multiple displays on the output side of the splitter.  It will just see a single HDMI output, so you can set your built-in display to duplicate to that "single" HDMI output, and the end result will be all displays showing the same content.  An HDMI splitter is cheaper than a dock too, and you won't have to deal with the drawbacks of the "indirect display" technology called DisplayLink that gets used by Dell's Dxxxx model docks, including the quite old D3000.

Thanks for the info. Sounds like a Splitter is the way to go.