Why is Brynden Rivers called the three-eyed crow? I know that he is/was a man of the Night's Watch. That explains the crow part but where was the three-eyes part explained?

Why is Brynden Rivers called the three-eyed crow? I know that he is/was a man of the Night's Watch. That explains the crow part but where was the "three-eyed" part explained?

Why was Brynden Rivers called the three-eyed crow? I know that he is/was a man of the Night's Watch. That explains the crow part but where was the three-eyes part explained?

Why is Brynden Rivers called the three-eyed crow? I know that he is/was a man of the Night's Watch. That explains the crow part but where was the three-eyes part explained?