Graphic Cards :: Win 8.1 - Multiple Video Cards Locking Up

I'm building a system for cryptocoin mining, and am having a little problem with getting Windows 8.1 (home, not pro) to run the 3rd card (GPU) My equipment is:

Gigabyte 990 FXA UD3 MOBO
AMD Vishera CPU
2x4 gig Corsair Ram
3x Sapphire r9 280x Video cards
1000w Corsair PSU
Scandisk SSD

All parts are brand new.

Here is the problem... everything boots just fine and runs with 2 of the GPU's installed. I have the latest AMD 13.12 drivers, and the AMD APP SKD installed. When I add the 3rd GPU though, windows will auto detect it as a Radeon 7900 with the caution sign, AND/OR a r9 200 series card, also with the caution sign, most times BOTH. Then it randomly freezes the system. A couple of times I got it to run long enough just by luck I guess, to install the 13.12 drivers on the 3rd GPU, but then in a few minuites the system would freeze again.

2 GPU's run just fine, the problem only shows up when the 3rd card is installed, and it happens on any of the 4 16x slots, or both 1x slots on the MOBO. It doesn't matter if the GPU's are directly on the MOBO, or on 1x16 risers. There are plenty of folks running this set up with up to 6 GPU's, so I know it can be done, but I must be missing something somewhere in the windows set up.