850W gold with three 760's?

Technically you should be fine. According to this, your system should pull less than 730W: http://us.hardware.info/reviews/4632/32/geforce-gtx-700-series-sli-review-geforce-gtx-760770780-in-sli-and-3-way-sli-energy-consumption. And in that review they were using an i7-3960x.

Personally, I would get a 950-1000W psu to be safe and not make the psu work as hard, but seasonic is top notch regardless.