The arrests were made from Sarai Mamrej police station area of the district's trans-Ganga region, about 40 km from here, and 19 sacks containing the tortoises were seized from their possession, police spokesman Uma Shankar Shukla said.
He said during interrogation the arrested trio - Pappu, Mahendra and Bablu - all of them residents of Sultanpur district, disclosed that they were planning to take the tortoises to West Bengal where they expected to get "a very good price".
A case has been registered against them under the Wildlife Act, the police spokesman added.