Third PCI-Express Slot on High End Z77 Motherboards can only be Utilized with an Ivy Bridge Processor
Apparently, The Third PCI-Express slot located on most of the High End Z77 Chipset motherboards can only work with an 3rd Generation Ivy Bridge processor. The reason behind is that the last full length PCI-e slot is wired with the CPU hence requires a Ivy bridge processor to operate, Many vendors had put a sticker on top of the slot which tells user to use an Ivy Bridge processor to run the third slot.
Further information can be provided through the above chart which shows that Ivy Bridge processors allow the CPU root complex to drive three devices splitting the PCI-e x16 links into following modes - x16/NC/NC; x8/x8/NC; and x8/x4/x4 hence the last x4 slot is wired to the CPU. In other words, Users aiming for 3-Way SLI/CrossfireX need to have a Ivy Bridge processor to run the third slot.