London--Publicis SA, the Paris-based advertising conglomerate, on Thursday said it had agreed to buy Chicago-based Bcom3 Group for $3 billion in stock, creating the world's fourth-largest ad agency. Publicis owns ad agencies Saatchi & Saatchi, Fallon Worldwide and Publicis Worldwide. Bcom3 owns ad agencies Leo Burnett, D'Arcy Masius Benton & Bowles, Bartle Bogel Hegarty, Ayer, Beacon Communications, LBWorks and other agencies. The agreement also brings in Japanese ad agency group Dentsu, which owns 21% of Bcom3, as a strategic partner. The partnership will give Dentsu's clients access to services provided by Publicis and will give Publicis' clients access to Dentsu's experience serving Japan. The partnership is expected to be similar to the one that existed between Dentsu and Bcom3, including initiatives such as pursuing new business together and merging some assets in certain geographic areas, said a Bcom3 spokesman.