The man, aged about 20, was one of five people in police custody as of Thursday evening as police continued investigating Saturday's attack.
The victims – aged 18, 23 and 24 and all wearing Jewish skullcaps – were attacked as they left a service at a Jewish school in the east-central city.
Police have said a group jostled and insulted the victims, then about 10 other assailants armed with hammers and bars joined in.
The main suspect denied having used a hammer, but authorities think he did, the source said.
Civil liberties groups and leading political figures including the French prime minister have condemned the attack.
The victims were recovering after being treated briefly in hospital.