Why is third monitor always mirroring one of my other two? I have a 2017 Macbook Pro connected to an HP Thunderbolt hub. From that hub, I have two USB-C -> DVI cables going into two monitors. For my third monitor, I have a DisplayPort cable from the hub to the monitor.

The two DVI monitors work correctly and show up in System Preferences &gt; Displays &gt; Arrangement. The third monitor (the DisplayPort one) does not show up in the displays. Additionally, it is always mirroring one of my DVI monitors.

What's interesting is that if I unplug one of the DVI monitors, the third monitor starts working correctly and shows up in the Arrangement tab. 

Is it not possible to have 3 monitors running where two are DVI and one is DP?



Edit: The resolutions of the two monitors using DVI are 1920 x 1080 and the resolution of the monitor using DP is 2560 x 1440.
The problem with your setup does not have to do with the resolution, as others have hypothesized. The problem is that the dock you're using is not supported on macOS for a 3-display configuration.

In practice, the HP Thunderbolt docks require MST-support for multiple displays from the system before they can enable 3 separate displays connected to the dock. MST is not supported on macOS for this purpose(in contrast to Windows), as thus it simply won't work.

You can connect two of the displays to the dock (as you have already figured out). The third display you'll need to connect directly to the MacBook Pro in order to get it as a separate display.