Well you dont need 3x 7970. 2 Would be enough for 3 Monitor gaming . (only if you need everything cranked and 60+ fps)http://www.tomshardware.com/reviews/radeon-hd-7990-devil13-7970-x2,3329-7.html
And if you go with 3 you need more W . They need 562W + on game peak. So add the other comp and you are high with W.
I suggest getting (for more $ ofc) Seasonic X-Gold 850 / Xfx Core Pro 850.
And the beast Corsair AX 860i (250$).