One is just a slightly different version of the original, one is an alternate transmog that does look different, and another one is the PvP transmog. Seriously, is this it? They showed so many more of them when they introduced Legion, I guess this is one more thing we can add to the list of things scrapped.
This is the interface they showed in the Legion trailer at Gamescom.
You can see the base skin and then 4 more skins. But in the game we only have the base skin and 3 more skins. That's 1 per artifact weapon that they promised us that's now missing.
Here are more pictures, you can clearly always see the base skin and 4 more skins.