The most descriptive is the second one. The first one could be wrongly interpreted as "export this file" and the third one is not so descriptive, although it looks great! Maybe you can use it for another action, such as reloading the file or something.
The first reminds me too much of the share (Contractor?) and undo icons, the third, while pretty on its own, looks too much like refresh. So I think the second/middle one shows that you're reverting the document the best. It's more like the New Document icon (which has the page and the plus, if I recall), whereas the third one is more like the more general "add" (plus) icon, where it doesn't have the document behind it.
Well they wouldn't* really but icons should be unambiguous and in my experience an arrow in a loop tends to mean refresh whereas an arrow pointing to the left usually means undo or go back. Neither are completely unambiguous but I think the first one makes the most sense as a revert icon.
*Except perhaps if they had the same document open in two programs for some reason.
I agree with this. For me the first arrow resembles going back, while with the second and third point to the current situation. Which one is the best to use depends on the save icon. I think it's good if the "Back to last save" resembles it a little. That way it clearly states it's use each time. I like the document behind the arrow so I would keep using that.
Suggestion for icons: Save: Document with yellow star. Undo: First Icon shown above. Revert to last save: Combination of star and arrow pointing back. But I don't know if you can make this look clean.