The body with cut marks bundled in a sack was found on July one from a deserted paddy field under Jagatpur police station on the outskirts of the city after she went missing on July 29, police sources said.
Satyabrata Sahu (16), his friend Ramesh Kandi (16) and a gardener-cum-watchman Bisweswar Mahakud (30) were arrested, the sources said.
They were arrested under section 364 (kidnapping), 368 (keeping in confinement), 376 (rape), 302 (murder) and 34 (common intention) of IPC, the sources said.
Police said Satyabrata took the victim to a farm house where she was allegedly raped and killed on Saturday night by the three.
The murder weapon was seized from the farm house.