The shop, based on a barge in London's Hoxton, is styled as a creative studio that will "build brands that capture the public's imagination".
Elliott and Williamson take on creative duties, while Brookes will be responsible for business development.
Fabula will work with a network of multidisciplinary partners from non-traditional creative, design, technology, media, entertainment, programming as well as business backgrounds.
The agency will work on the brand redesign for the personal training company Body Studio and the July launch of the Superhuman exhibition at the Wellcome Collection.
Elliott and Williamson left WCRS in May 2011 after more than five years. Before that, they were at Mother, where they created the Orange Gold Spot "film funding board" campaign and worked on Coca-Cola, Super Noodles and Dr Pepper.
Brookes has been a freelance communication consultant, working with ITV, since October 2011. She was at Wieden & Kennedy before that, joining in 2001 as the global group account director.
She was promoted to client services director in 2005 and then to managing director of the research and development arm Platform in 2009.