What exactly happens. Windows doesn't find the third card? What is device manager showing? Are you using PCI-E 1 to 16 extension? Is it possible that some of the 6-8 pin power connectors for the VGA's are not exactly the same? Like some taking power from a Molex and not straight from the PSU or 2 pins sharing one cable which is splitted into two? I've had an R9 290 not working with 350W PSU Fortron because it simply didn't liked the 6 pins for VGA or wanted specifically 8 pin and not 6 pin VGA power connector in the 8 pin slot.
Nothing happens. That's the issue. It's not in device manager. It's as if the card does not exist. The only existence that it shows is the spinning of the fan itself. That's it.
I'm not sure what kind of extensions I was using before. I don't think it's the 16 extension as the end that I am plugging into the computer is rather short. At the moment, I plugged the other two directly into the computer and still nothing is happening. I downloaded Ubuntu thinking it was an Windows issues and getting the same result. What the heck is going on.