This is your board, Am I right?
I've checked your reference image, question is: did you installed the cards just like in the image (same PCI-e Slots)?
If so, try to move them one slot above where they sit (PCIe-1,3 & 5 instead of 2, 4 and 6) to check if you got a bad PCI-e slot.
Alternatively, just try all the slots using a single GPU and see if it can be detected.
I've checked your reference image, question is: did you installed the cards just like in the image (same PCI-e Slots)?
If so, try to move them one slot above where they sit (PCIe-1,3 & 5 instead of 2, 4 and 6) to check if you got a bad PCI-e slot.
Alternatively, just try all the slots using a single GPU and see if it can be detected.