In Greek mythology, the Fates are the literal spinners of the thread of life. They spin the amount of time each individual will live, and then slice that thread short when it's time for that extended vacation to Hades. In most of the books I read, the Fates were old, haggard, and usually very cruel when confronted. The latter may still apply, but in the sequel Percy Jackson and the Olympians: Sea of Monsters, 'old and haggard' definitely doesn't fit.
THR reports the Fates will be played by Mary Birdsong, Missi Pyle, and Yvette Nicole Brown in the follow-up to 2010's Percy Jackson and the Lightning Thief, which has the son of Poseidon on a quest to find the Golden Fleece and rescue his best friend, Grover. Logan Lerman, Alexandra Daddario, and Brandon T. Jackson, and Jake Abel are expected to return. Thor Freudenthal(Diary of a Wimpy Kid) will direct, with Chris Columbus producing.
Pyle and Birdsong most recently appeared in two Best Picture nominees at last weekend's Academy Awards. Pyle had a small role in The Artist, while Birdsong played a family friend to George Clooney's character in The Descendants. Brown is a regular on NBC's hit comedy, Community, and also appeared in 2009's Hotel for Dogs, which Freudenthal directed. There's still a lot of casting to take place, but the film is still a year away from release.