Condé Nast is the publisher of flagship media such as Vogue, Vanity Fair and The New Yorker. Shortlist Media, is the creator of a "freemium" based model for Stylist, Shortlist, Mr Hyde and Emerald Street.
Redwood, is an international content marketing agency working for brands such as Land Rover, Virgin Media and lastminute.com.
The three new publishers join as members alongside the original founding partners – Bauer Media, Dennis Publishing, Haymarket Media Group, H Bauer, Hearst UK, Immediate Media and Time Inc UK.
Magnetic has been created to help advertisers get the very best return from magazine media and to better understand the evolving role and influence that the content and channels play in today’s multi-platform environment.
Nicholas Coleridge, chief executive of Condé Nast, said: "Magnetic is a bold and potentially important statement for the magazine industry, and we are pleased to lend support to it."
Sue Todd, chief executive of Magnetic, said: "Condé Nast, Shortlist Media, and Redwood are great additions to Magnetic.
"To have businesses around the table, which place such high value on, and are so skilled at developing inspiring content, will add great value to the conversation with advertisers.
"Our job is to better show the return that can be gained from investing in some of the most trusted and valued content spaces in UK advertising."