Display configuration problems with Mac Pro I have a Mac Pro (trash can) with dual D600 graphics connected to 3 x 4K displays using mini-DP to HDMI cables that are rated to work 4K @ 60Hz.
Due to the size of the mini-DP connectors, I can fit them side-by-side, but not directly underneath each other. The following diagram demonstrates how the displays are connected to the display ports in the back of the machine.

    [1]    [2]
    [ ]    [ ]
    [3]    [ ]


Problems I'm currently facing

Only two of the three displays is being detected at a time. If I pull out cable 3, display 2 starts working. If I plug cable 3 back in, display 2 remains active and display 3 remains black until I reboot, then they go back to 1 and 3 working and 2 remains black.

None of the displays are working at 60Hz, though they support 60Hz through HDMI and DisplayPort.


Are there any tips, tricks or tools I can use to identify the problems and manually configure the displays to work correctly?
Apparently, there are three buses on the Mac Pro, which can handle either two normal (p.e. Full HD) displays or one 4k display.
[1]     [2]
[1]     [2]
[0]     [0]

The numbers indicate to which bus those ports are connected. Moreover, the Mac Pro has got an HDMI output, which is also connected to bus 0.
Apparently however, if you want to get three 4K displays to work, you have to plug one of them into a port from bus 1, one into a port from bus 2 and one directly into the HDMI-port for some reason. (I assume that this will not be a problem as you are using Thunderbolt to HDMI adapters anyway)
For more detailed information, please take a look at: How do I connect a 3rd 4K monitor to my 2013 Mac Pro?
Good luck!