East Champaran Senior Superintendent of Police (SSP) Jitendra Rana said all the three arrested boys were minors.
They and their murdered friend Ashish Kumar, a student of class IX, had formed a group called 'X-gang', he said.
"They had stolen around a dozen motorcycles. Their crime was about to be revealed or busted and they suspected Kumar for this. They killed him by slitting his throat and cutting his wrists after calling him to a deserted house," Rana said.
The deceased student's maternal uncle had registered an FIR against eight friends of Kumar and a search was on for a few more students in the case, he added.