What does 3x retina mean?
1x: Standard Definition
2x: "Retina" Definition
3x: "High(er)" "Retina" Definition


They've added more pixels than needed, because the screen resolution for the size of displays they use is already "retina". Which means that you can't see any individual pixels from a certain distance off of the screen. For iPhones it's like 30cm I believe.

Now that they've added more pixels then needed, they are capable of maximizing brightness and color ratio's, as well as contrast.

Update: Apple seems to be using a default resolution of 568x320 for assets (pictures, mostly). I suppose they call this retina for assets, so 3x retina would enlargen the picture 3 times, which makes 1704x960. I'm not quite sure about why this resolution is specifically chosen, but it might have something to do with the new 1080p display on the iPhone 6 Plus