Law & Order: SVU veteran Mike Doyle - who has also appeared on Oz and A Gifted Man - will first appear in episode two, playing Frank Alpern, nemesis of Gavin (O'Quinn).
Aubrey Dollar (Weeds) will make her debut as Annie, an aspiring journalist, in the show's fourth episode, TV Guide reports.
The third cast addition is Mili Avital (Damages), who will appear as a single woman who yearns for romance but is hiding a dark secret. The actress most recently starred in Prisoners of War, the Israeli drama that inspired hit US series Homeland.
666 Park Avenue also stars Vanessa Williams (Desperate Housewives), Dave Annable (Brothers & Sisters) and Rachael Taylor (Charlie's Angels).