Sorry, now I slipped up. I got my Harrisons mixed! :)
Well, Hoover also outlived his wife, I'd forgotten. Hoover lived into his 90s, I think. Benjamin Harrison served as President after the Civil War, though he was born prior to it (he served in the Civil War). So I guess it depends on how you define "since the Civil War." I think. Now I have to verify this myself....
Benjamin Harrison did outlive his wife. So I guess the error was in the wording of the question. Or you could say the three are Nixon, Hoover, and Harrison. Wilson outlived his first wife, but not his second.