Three of the four, all members of an elite police unit that fights gang crime, were brought before a magistrate Saturday after being taken into custody .
The fourth officer detained in the case was expected to be released on Saturday.
The policemen initially denied having sex with the 34-year-old Canadian woman, who arrived in France on holiday in mid-April.
But one of them admitted overnight to having had consensual sex with her, the source said.
The woman's allegation sent shockwaves through the headquarters of the Paris police, which is widely known by its address -- 36, Quai des Orfevres -- and which is where the alleged gang rape occurred overnight .
According to judicial sources, the woman met the officers during a night of heavy drinking at an Irish pub frequented by many police from the headquarters.
They said she agreed to follow them to their headquarters, just across the Seine from the pub.
She registered a criminal complaint soon after the alleged assault, they said.
Initial medical exams were carried out and DNA samples were taken but the results were not yet known.
France's national police inspectorate (IGPN) -- which is carrying out an investigation in parallel with the criminal probe -- has searched the officers' homes.