The boys, both school students, were abducted from Dimapur on July 28 and killed on August 3 at a place between Diphu and Lumding in Assam, Special Branch DSP Israel Xuivi said.
They then telephoned the father of one of the victims, a student of class-VIII, seeking ransom and were arrested yesterday when they came to collect the money at Dimapur railway station, he said.
Of the three, two were from Assam's Karimganj district and the other one from Bihar. They were staying at Dimapur, the DSP said.
During interrogation, the three admitted to their crime and also told police that they were involved in a number of kidnappings and murders for ransom in Dimapur, he said.