Emergency services in the northwestern region of Castile and Leon said they had responded to a blaze in the town of Miranda de Ebro on Tuesday night.
Two people were found dead at the scene and a third person died in hospital, while two children aged seven and 11 were among the injured, the emergency services said in a statement.
The suspect handed himself in to police on Wednesday and is in custody, the central government’s top representative in Burgos province, Pedro de la Fuente, told reporters.
Interior Minister Fernando Grande-Marlaska spoke of “an utterly despicable” case “that we can say is gender violence”, with the three dead women including the suspect’s ex-partner, her mother and another resident.
Daily newspaper El Pais, citing sources close to the investigation, said the women were a 24-year-old Colombian and two Spaniards aged 58 and 78.
The suspect was already known for having committed other crimes, added Jose Carlos Donoso, the national police’s most senior office in the province.
Spanish media reported he had previously served prison terms for violence against a woman and sexually abusing a girl.
The central government’s office against gender violence wrote on X that it was gathering information on the case.