The inmates, identified as Arun Mahto, Shashi Mahto and Kamal Mukhiya, had been lodged in the jail for many years and had been booked for various cases, Samastipur Superintendent of Police Varun Kumar Sinha said.
Searches are on to arrest them, he said adding security at the divisional jail and a sub-jail at Dalsinghsarai have been beefed up since the incident.
The role of a homeguard jawan is being investigated for the jailbreak. Strict action would be taken against him if he was to be found guilty.