You definitely don't have enough power for 3 290s. Trust me I have 8 of them each one draws ~300 watts and the rest of your components are an easy 50 watts. That could lead to inconsistent hash rates when running multiple cards. Also like somebody else said the cards are all different, you might be pushing the powercolors too hard/or not enough causing a smaller hashrate.
Holy crap, I just measured the rig at 1100 watts total with my new meter! Each card is drawing roughly 320 watts, so you're right on the mark. I've been pushing that corsair 850W supply pretty hard... I'm actually amazed it has worked this well. Surprisingly, the supply feels cool to the touch and the fan is barely spinning. Might be time for a higher-wattage supply or some other setup to split the load.