I have single gtx 760 3gb and still have room for another gpu. Should I go for a newer like 970 or 980 or get SLI? How better is 980 from 760 sli?
Get something newer. Your card has 192-bit Memory Interface Width, and a memory bus that slow is not going to handle 3gb's of memory very efficiently, even with a single card. You would be in a much better position with a single 970 or 980. While the amount of VRAM might be greater with the two 760's, you won't see better performance than you would out of a single 970 or 980. And you'll also have to deal with the first four problems I list in this answer here. Let me quote myself:
First, you won't have to fuss around with getting SLI to work properly. Not that its a massive pain, but it is a pain.
Second, some games do not even support SLI properly. Now this may not happen often, but imagine your shock when your $1000+ GPU setup won't play nice with certain games. (Don't get me wrong, I know most games support SLI, but this doesn't change the fact that there are some compatibility issues with certain games...)
Third, power consumption. It may not be a huge issue, but its another issue to stack on-top of the other issues that come with running two cards. (~150 watts per 970, vs ~250 for a single 1080)
Fourth, resale value. Not that you'll want to sell, but this is still something to think about.
All of these issues apply directly to your situation as well. Except the power consumption would be slightly different for 760's vs 970's; but the same concept applies, you'll be drawing significantly more power.
You would also have to find a 760 3gb card, which would be tough to get new, so you would be stuck with used or refurb models off of eBay.
So all in all? Don't go for SLI, instead of searching for the same card, save your time, put more money up front and get yourself a GTX 970 or 980 (go Ti if you can!).