Cara Theobald will play a new kitchen maid called Ivy in the Carnival Films period drama, while it has also been confirmed that Lucille Sharp has joined as lady's maid Miss Reid.
Last week, it was revealed that Matt Milne, the star of War Horse, had signed up to play a footman named Alfred.
In January, Oscar-nominated actress Shirley MacLaine was brought in to portray Martha Levinson, the mother of Lady Grantham (Elizabeth McGovern). Sharp's Miss Reid is to serve as Martha Levinson's maid in the Julian Fellowes-penned piece.
Downton Abbey is expected to return with a new series on ITV later this year.
Watch some of the Downton Abbey stars talking about the third series to Digital Spy below: