Honestly with a GOOD
700w psu you have enough power to run 3 680s http://www.guru3d.com/articles_pages/geforce_gtx_680_review,9.html
Yours however to put it bluntly a horrid PoS. I would replace it, even a 500w model is more than enough for a 680 + any cpu. Since you have the cash for a 680 you really should replace it man. You could also drop down to a 670, overclock that, then spend your savings on a psu if you absolutely have to stay in budget.
170w x 3 = 310w + 190w (lets assume heavily OC'ed i5 3570k) and thats only at 400w. (lets then assume +50w for drives and fans.)
Edit: according to qbsinfos benchmark link it appears a 700w might not have enough power to run 3x 680s but you get my drift nonetheless.