One of TV's greatest shows is about to get even better! Oscar-winning actress Shirley MacLaine has signed on to join the cast of Downton Abbey for season three of the hit PBS show. Carnival Films and ITV, producers of the series, announced the news today, saying Shirley will begin filming scenes next month. OK! GALLERY: THE STARS COME OUT IN STYLE FOR THE 2012 SAG AWARDS  She will be playing new character Martha Levinson, who is Lady Grantham's mother. Carnival Films' managing director, Gareth Neame, says her character ... Read More >>