The three were arrested early on Monday morning and kept in detention as part of an investigation into the murder of a 32-year-old French national born in Burkina Faso who is identified as Béli N in local media.
He was beaten to death by a gang of a dozen adolescents - some as young as 12, according to witnesses.
The attack took place on a playground in front of picnicking families on a disadvantaged housing estate.
Assailants used a plastic chair leg and a piece of wood.
The victim, who had recently arrived in the area, had a criminal record for robbery and drug-trafficking in the Paris region but had not attracted the police's attention since the beginning of the year, local prosecutor Cécile Gensac said on Monday.
"No connection between his past and what happened on Friday evening in Pau," Gensac said.
Pau mayor François Bayrou, who is the leader of the MoDem party which has joined President Emmanuel Macron's government, condemned "very serious, tragic and criminal events", pointing to young people "often of foreign origin" involved in petty crime based on drug trafficking.
Police on the housing estate where the attack happened is to receive reinforcements in September, Interior Minister Gérard Collomb pointed out.
The attack comes as Macron's government prepares a plan to tackle entrenched problems in underprivileged areas of French cities.