By Francesca Landini and Andrés González
The three companies will set up a special purpose vehicle (SPV) next week, which will become the owner of Abertis, three sources close to the matter said.
"Atlantia, ACS and Hochtief will create a vehicle and give it the financial resources to buy Abertis' shares from Hochtief," one of the sources said.
The deal, worth more than 17 billion euros (15.08 billion pounds), will create the world's biggest toll road group and also make Atlantia a shareholder in Hochtief.
A compromise in March between the Benetton family, which controls Atlantia, and Real Madrid soccer club president Florentino Perez, who heads ACS, put an end to a bidding war between the two companies over Abertis.
Builder ACS, construction company Hochtief and Abertis declined to comment, while Atlantia could not immediately be reached for a comment.
Under an agreement reached in March, Hochtief took over Abertis to transfer it to the SPV, which will be majority owned by Atlantia.
Atlantia is under scrutiny after a bridge operated by its motorway unit Autostrade per l'Italia collapsed last month, killing 43 people.
The disaster sparked public outrage in Italy with Rome's anti-establishment government piling blame on Autostrade, which runs a section of A10 motorway that links Genoa to the French border.
The Italian government has vowed to revoke Autostrade's Italian motorway concessions, saying the company reaped extra profits from the motorway business.
(Additional reporting by Jose Rodriguez, Matthias Inverardi and Stefano Bernabei; editing by Agnieszka Flak and Kirsten Donovan)
By Francesca Landini and Andrés González
