Asrock H81 ProBTC & GPU Slot3 Defect?

Hi all,

Trying my luck again.

I have an Asrock Pro H81 ProBTC and my OS is Windows 10
Windows does not recognise any GPU in its 3rd slot from the left, as shown below:
https://drive.google.com/file/d/0B6HNx74FwQi2WGRQQVRJcFdBVEk/view?usp=sharing

With the monitor connected to the GPU, Windows 10 starts but the GPU will not be recognised in Device manager.
I have tried this in another 2 motherboards (H61 ProBTC) and the result is the same, except the 4th slots don't work.

Have tried 6 GPUs on the third slot. All work but aren't properly recognised by Windows. All GPUs work on other slots.
I have updated to the latest Bios. I have also rest CMOS. I have gone into 'troubleshooting' to detect hardware problems.

These are all the solutions I have found in other forums and now I am out of ideas.
Would be eternally grateful for your help.

Cheers,