How do I connect a 3rd 4K monitor to my 2013 Mac Pro?
The main article on 4k displays links to the below article on Mac Pro:


https://support.apple.com/en-us/HT206587
https://support.apple.com/en-us/HT202801


The trick for the third display on Mac Pro is to use 2 Thunderbolt and one HDMI cable to connect each directly to the Mac Pro. Basically each bus can handle 2 normal displays or one 4K display each.


  Three 4K displays: two connected via Mini DisplayPort and one connected via HDMI


You are correct about bus 0 conflicting with the HDMI adapter as well.



If you're not on the 2013 Mac Pro - see this thread for options for "unsupported" adapters that might work for lower resolutions or lower refresh rates than the native ones.

How can I run 3 4K monitors on my 2015 MBP