A 960 will only be better if the game doesn't support SLI, because it's faster than a 770. It's not as fast as 2 770s, though. A 960 will do great on a single 1080p monitor, but I wouldn't think much of its performance pushing 3x that.
The 960 is not a 4k card, and 4k is only 25% more pixels than you're pushing now. I think you will be disappointed in it running 3 displays.
The Jedi Master