My setup is as follows.

Dell XPS 15 9550 with Intel HD 530 and NVIDIA GeForce GTX 960M
Dell WD15 Thunderbolt 3 dock
Two HP 2159m 1080p 60Hz monitors, landscape orientation
One ASUS VG248 1080p 144Hz monitor, portrait orientation

One HP monitor is on the laptop's HDMI, the other is on the dock's HDMI. The
VG248 is on the dock's mini DisplayPort connection. This setup is capable of
running 3 displays (including laptop display, which I have disabled) and the
exact same setup works correctly on Windows 10.

They are aligned like this:
          |---|
          | A |
          | S |
|----|----| U |
| HP | HP | S |
|----|----|---|

To produce the crash that I'm experiencing:
1. Set both HP monitors to 1080p on "Auto" refresh rate
2. Enable the VG248 in portrait mode at "Auto refresh rate"
3. At this point, there will be no signal on the VG248, but it will be shown as
enabled in the display configuration. It has generally only received signal
when I set it to something below 60Hz (but in a different order than shown
here)
4. Set VG248 to 59.94 Hz
5. System hangs entirely, requires a hard reboot

It seems that the problems mostly only occur with the 144Hz monitor, so I would
suspect that it's a bug involving that. The dock can only support this monitor
setup at ~60 Hz, so that probably explains why the 144 Hz display doesn't get a
signal until I lower its frequency. I've noticed that KDE's display settings
seem to revert to "Auto" refresh rate occasionally, which usually forces me to
crash my system trying to fix it.