How to connect 3 monitors to a Late 2013 Mac Pro?

I wonder what I am missing here?

I am using three non-apple monitors. 1 4k Visio, 1 Gateway and one Insignia monitor. Two of the monitors only run 1080 and the third runs 4k out of the hdmi output.

The 4k monitor only works via the HDMI output 'only' on the Mac Pro and doesn't work well via any thunderbolt conversion... Not only does it not work at 4k, but it runs at 1080 and flickers off and on every so often.

All the monitors are connected via different Bus's as according to the website link below.

I can get two to work anytime for the most part and I can hot swap plug and unplug and the monitors all seem to work, but never more than two. No monitors work at all if I try to adapt the 4k monitor via thunderbolt.

I have tried using both Thunderbolt(display port) to HDMI and Thunderbolt to DVI in various combinations with no luck.

This website is relatively unclear, though I may be missing something.

This page was not helpful and it was written specifically for my computer.

Use multiple displays with your Mac Pro (Late 2013) - Apple Support

Am I missing something?