In a statement from Hopscotch this morning, the firm said the three parties were in "exclusive negotiations" to form the alliance, through which Hopscotch Groupe and Comexposium would each have a minority stake in Sopexa.
The negotiation process is due to end on 29 September 2016, the statement added.
Hopscotch, which is listed on the Paris Stock Exchange, employs 540 people through its own branded agencies and specialist shops including Sagarmatha, Human to Human, and Le Public Système PR. It generated turnover of €150m (£126.1m, $166.7m) in 2015.
Sopexa, which specialises in food and lifestyle PR, owns 26 agencies across the world, employing 280 people in around 60 countries. Comexposium organises more than 170 b2b and b2c events globally in industries such as food, agriculture, fashion, construction and transport, with offices in 30 countries.
Hopscotch stated: "The purpose of this alliance is to create the largest French PR network worldwide, capitalising on the specific areas of expertise of each of the three companies.
"As such, this alliance will represent the only alternative to the Anglo-Saxon networks to design and highlight what France does best: food, luxury, tourism, and lifestyle.
"It is a coherent industrial project for the three companies, setting the course for excellent growth prospects."