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.
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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?