Winblows: System wont see 3rd GPU? Any advice?

[Rolf]

Then I am pretty sure it's a hardware issue.
Triple check all the cables coming to card 3.
Does it gets required power from PSU ?
Does its fan spin ?
Is there some kind of switch to turn off PCI-E slots ?
Is card 3 fully inserted into the PCI-E slot, with nothing blocking it ?

Oh, did you try leaving only card 3 in its PCI-E slot ?
You know, the motherboard may be faulty.