Havent read the entire thread, but isnt 750W a bit low? That CPU alone at that clock could be pulling 150-160W at stock voltage (and possibly considerably more if you overvolted it). Add the motherboard, RAM, water pump, fans, and you are probably >200W, leaving 550W for the three cards. YOu dont mention overclocking the GPUs, but even at stock speeds thats cutting it close IMO.
It's right at the high water mark running everything at full tilt, so yeah it's a little undersized. With all three cards mining and prime95 running my killawatt reads 760-780W at the wall.
But, I don't really mine with this rig, and have yet to actually stress the system running any of the games I've tried. Generally it tops out about 430W on my killawatt playing Skyrim, 550-600W playing Witcher 2 which actually uses all 3 cards a little more.
If I did mine, it'd be in linux without the 100% CPU bug, I'd expect to see ~600-630W at the wall with just the GPUs spinning.