According to the UK paper Daily Mail a further three cast members have joined Prometheus, Idris Elba, Sean Harris and Kate Dickie:
“Elba will be best known to CBMers as playing Roque in The Losers as well as Heimdell in Marvel’s upcoming Thor of course. Harris you may recognize from Harry Brown with Michael Caine, or from the original Red Riding trilogy with Andrew Garfield. Scottish actress Dickie will most likely not be well known to US readers but she starred in the bleakly brilliant Red Road in the UK. “
The above quote was taken from the Comic Book Movie website. Nothing is known about the characters they play. Thanks to Invisible Darkness and Mathew Kirk for the news.