The incident occurred when they were crossing Narjora 'chaur' (water body), where one of them slipped into deep waters, and the two others also drowned in a rescue effort, Kamtaul Police Station SHO Kundan Kumar said.
The deceased have been identified as cousins Vikas Kumar (14) and Kajal Kumari (13), and Uday Shankaer Rai (15), he said.
Local villagers fished out the bodies, Kumar said, adding, they have been sent to Darbhanga Medical College and Hospital for post-mortem.