You can make a great case for all three. Not sure I'd agree about Kepler having far better offensive stats than Rosario, and there's quite a gap between those two and Buxton offensively. Kepler is probably the biggest snub to me. He's been very good defensively in addition to the patience and slugging. Looking at his swing, he looks like he's going to be a superstar. So compact and quick, and lately pitchers are making the pitch they want and he's still crushing it. As for Buxton, he's getting there for sure. I think between the newly acquired vets and the emergence of Rosie, Polanco and Kepler have taken a ton of pressure of Buxton at the plate. Sano looks terrible at times, but amazingly his OPS is almost identical to Kepler.
Far better was an overstatement when comparing Kepler to Rosario. Better, yes, with that additional 40 pts of OPS but "far" was an overstatement.
The gap between Buxton and Rosario in terms of OPS is less than the gap between Kepler and Rosario FYI. I think the only way you can justify Rosario is the fact that Buxton has missed some time with injury. That said, things like WAR that factors that in still like Buxton over Rosario.
Buxton: 2.3 F-WAR 2.8 B-WAR
Rosario: 1.3 F-WAR 2.3 B-WAR