Gabriel Fortin, 48, was arrested in 2021 in the southern city of Valence.
Over the previous days, two human resources managers who helped sack him years earlier had been shot dead.
The third victim worked at a job centre. Mr Fortin, dubbed by media as the "HR killer", is also accused of trying to murder another manager.
An unemployed engineer at the time of his arrest, he has refused to speak to investigators since then.
The first killing happened on 26 January 2021 in the Alsace region of eastern France. Human Resources manager Estelle Luce was shot in the head in her company car park after work.
Later that evening, about 50km (30 miles) away, another HR manager was shot at his home by a man posing as a pizza deliverer. The victim, Bertrand Meichel, survived.
Two days afterwards, 500km to the south, a man wearing a facemask and carrying a white plastic bag entered the Valence local job centre, pulled a gun from a plastic bag and killed the benefits director, Patricia Pasquion.
Minutes later another HR manager, Géraldine Caclin, was shot dead at an environmental services company near Valence.
The number plate of the car used by the gunman as he left the job centre led police to Mr Fortin. He was immediately linked to the later murder.
In 2009 Ms Caclin had led dismissal proceedings after an unsuccessful trial period. Mr Fortin had then registered with the Valence job centre, and eventually his unemployment benefits ran out.
Ms Pasquion never dealt with him, but police believe he held a grudge against staff at the centre.Investigators quickly connected him with the earlier shootings in eastern France.
Estelle Luce and Bertrand Meichel had been involved in his dismissal from another company in 2006, more than 14 years earlier.
Police spent more than two years combing through data on his computer. They say they have extensive evidence of his enduring bitterness, as well as efforts to track the movements of his eventual victims.
Gabriel Fortin appeared in court in Valence on Tuesday accused of three murders and a charge of attempted murder.
Ahead of the trial, Ms Pasquion's sister told France's Europe 1 radio: "He was armed and facing defenceless women... He never tried to speak or listen. He just killed. It's pure cowardice."
