This is the slot configuration of this motherboard:
1 - PCIe 2.0 x16_1 slot (single GPU 16x, dual GPU 8x)
2 - PCIe 2.0 x1_1 slot
3 - PCIe 2.0 x1_2 slot
4 - PCIe 2.0 x16_2 slot (8x only, 2nd GPU)
5 - PCI
6 - PCIe 2.0 x16_3 slot (black, 4x or 1x only)
The manual says your's doesn't have it, but some BIOSes have a PCI Express configuration menu as detailed in the screenshot below, you will need to go in and specify that the video card slots are 8x, and other GPP slots to be used for video cards are correct width (1x or 4x for the black slot, 1x for others) AND some boards have a BIOS option such as "GPP Slots Power Limit" which can be set to 75W power (some boards default to 25W power, not enough for a video card). Without this option, general purpose slots may be power limited:
There is a motherboard switch called "EPU Switch", it's default is disabled, but in enabled it "intelligently monitors power consumption". Switching it to the other option may help.
There is an advanced BIOS option "initiate graphic adapter" this should be set to PEG/PCI (PEG: PCI express graphics first).
The next thing is to cable each card to real monitors (or at least cable each to the same multiple-input monitor). This test is even better than dummy plugs. The cards may not start without a monitor connected.
This motherboard uses the AMD 990X chipset and SB950 southbridge.
Whereas the 990FX
allows 2x16 or 4x8 PCIe, the neutered 990X
allows 2x8 PCIe crossfire, and has additional PCIe ports that are not specifically for video cards.AMD 990X Chipset
The ideal foundation for your ultimate PC
With support for up to 2 AMD Radeon™ Premium Graphics boards with AMD CrossFireX™ technology and advanced technology like SATA 6Gb/s to accelerate your connectivity, the AMD 990X chipset is an ideal fit for your FX processor and your ultimate PC.
Here is the block diagram for the FX chipset, I wasn't able to find one for the "X":
Here is a comparison of features:
Note the number of PCIe channels, 42 vs 26. This indicates that one of the 16x channels in the block diagram is removed from the northbridge, but other PCIe details are likely the same.
Regarding shorting hot-plug pins, this shouldn't be required in the black slot, all PRSNT2# hot-plug pins on the motherboard should still be wired to detect a card (pins B81, B48, B31, B17). Detection only needs to be forced by shorting (B17 to B16 ground) when using an adapter such as a 16x to 1x. If nothing else works, a 16x->1x with molex power and the pin shorted will be the final option to try to get a third GPU working.