Your system is a great system right now. Besides that fact, I would not recommend 3 way sli on that motherboard. Yes you can do it, but the third card will be running at a reduced speed and seeing as 3 way sli does not scale too well it would be a waste of money if you bought a new PSU. The performance gain would be mediocre compared to the extra price of a new PSU, and extra heat the third card would put out. Sell the card and then save the money or use that money to get a better screen or something? This is all personal opinion and remember, in the end its up to you.
I don't know if 3-way SLi is worth it, the furthest I have ever went is 2-way SLi and I have small concerns about that because even with the highest performing graphics cards I still get micro stuttering (frame pauses) which affects my gaming experience. You'll need to decide how much the little niggles in your 2-way setup annoy you.
PSU, google is god. You psu has cables to support 3-way SLi but I have misgivings about using a 850W supply for 3-way SLi. I'd be looking for a 1kW supply. I've used a 850W supply before for a pair of GTX 580s in SLi and an overclocked i7-3930K, it ended in an unstable system, lots of crashes and a few major upgrades to get it stable again.