The narrator, the narrator's wife, Robert
The narrator is the most obvious protagonist of the story. He might not be the most likeable character, but he's the person telling the story, and seems to change the most in that story. His actions, specifically, the act of trying to explain a cathedral to Robert, drives the story toward its conclusion. But, all three of the main characters could fit the protagonist role. If the events were being narrated from the point of view of Robert, and focusing on his changes, or the woman, and focusing on hers, we would still have the same basic story of a person who starts of in a bad way and ends up experiencing something extraordinary.