Vostro 5590 + 3 external monitors

@aledbet3  Yep, it's a bandwidth issue.  The Vostro 5590 uses Intel Core 10th Gen CPUs that use the "Comet Lake" architecture, not the Core 10th Gen CPUs that incorporated a total redesign "Ice Lake" architecture.  The Comet Lake CPUs still use an older GPU that only supports DisplayPort 1.2/HBR2, not DisplayPort 1.4/HBR3.  And the Vostro 5590 doesn't offer Thunderbolt 3, which incidentally made the WD19TB a waste of money over the regular WD19 since if you connect a WD19TB to a non-Thunderbolt host, it operates in USB-C backward compatibility mode at reduced functionality equivalent to the regular WD19.  And when the WD19 (or WD19TB in backward compatibility mode) is paired with a system that only supports DisplayPort 1.2/HBR2 over USB-C rather than DP 1.4/HBR3, you're limited to dual displays up to 1920x1200 each, or in your case dual 1920x1080 with a tiny amount of bandwidth left for a not especially useful third display.

As for the D6000, the reason it's different is that the WD19 dock family rely on DisplayPort Alt Mode, which is a native GPU output wired to a USB-C port (or up to two such outputs wired to a TB3 port).  The D6000 on the other hand relies on "indirect display" technology called DisplayLink -- not to be confused with DisplayPort -- when it's only running two displays.  When running three, whichever display is on the HDMI output relies on DisplayPort Alt Mode and the other two use DisplayLink.  That's why it can run higher-end setups.  However, DisplayLink comes with some drawbacks that can be significant, which I've written about in the post marked as the answer in this thread.

So you've got two options here:

• Connect your third display directly to the Vostro's built-in HDMI output, which will work because in that case its bandwidth requirements won't have to be met over the USB-C DisplayPort Alt Mode link.  You'll have an extra cable to deal with, but you won't have to deal with the drawbacks of DisplayLink.
• Swap out the WD19TB for a D6000.

If you don't go for the D6000 though, you might still want to swap the WD19TB for a regular WD19 unless you just want the TB for futureproofing in case you get an actual TB3-capable system later.

And lastly, the D6000 only supplies up to 65W to the attached system.  The WD19TB supplies up to 130W, and the WD19 can supply either the same 130W or 90W.  You may want to check the power supply wattage on your Vostro 5590.  If you find it came with a power supply greater than 65W, then even if you do decide to get a D6000, you may want to keep the Vostro's own power supply directly connected as well in order to avoid things like slower battery charging and reduced performance caused by running from an undersized power source.