Bostock will remain on the B|Com3 board and consult on major client business for the company. Haupt was previously CEO of the Leo Group before the merger of MacManus and the Leo Group in February 2000 which created B|Com3, the world's seventh-largest agency group.
Earlier this year B|Com3, which has revenues of $2.2bn (£1.53bn) and billings of $17.9bn (£12.48bn), ditched plans to go public because of volatile market conditions.
The group, in which Dentsu of Tokyo holds a 20% stake, owns Leo Burnett Worldwide, D'Arcy Masius Benton & Bowles and the Starcom MediaVest Group.