Following GRRM’s most recent clues, fans scoured the Internet to try and figure them out. Now one of them has been found for certain, as the actress Esmé Bianco—burlesque and cabaret performance, singing, and modelling are among her other activities—has the news up on her official website.
For those wondering what guesses are for the other two parts, Jamie Campbell Bower, who is involved in the most recent Twilight and Harry Potter adaptions, seems surest as Ser Waymar Royce. The third role, the one GRRM said no one would guess, seems to have been cracked by Paul Gude and the rest of the sleuths over at Winter is Coming seems to have cracked it: Isaac Hempstead-Wright as Bran Stark, who is a member of the Kent Youth Theater where the news ticker is presently congratulating him for a “role in a new series”. A photo can be found at the WiC site, and more can be seen at the Kent Youth Theater page such as this one (scroll down a bit).