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.
That's a possibility. I assumed each draws about 250 W, but I have a power meter on order to actually test this. I have a radeon HD 7970 that I plan to run in a separate junk computer, but maybe I could swap that in instead of a R9 290... assuming it's actually lower-power under load. The Corsair supply is a good one, but I do doubt its ability to supply >900W continuously. I've tried adjusting gpu-powertune to various levels without any change in the results.