Could you do it? I bet so.
Would I do it? Probably not
At all components max load with an overclocked quad (~200w), you'd be almost exactly at 750 watts.
It won't do tri-sli, but I get about 600 watts for that setup with 2 cards. Add another 150 watts for the third card, and you're at 750 already. Almost all of the added load is going to be on the 12v rails too- I'm not sure how much that matters in your case.
I think that a lot of people on here tend to over-psu, but in your case, I'd really look for at least a little more power. The antec 850w is supposed to be excellent- I'd look at that.
BTW, if you're thinking about selling that psu, I might be interested. I'm really pushing mine too hard right now.