The clash occurred around 10am on Thursday when one group of persons was constructing a house on a disputed piece of land at Mohanpur and a rival group of claimants of the land asked them to stop work.
The three brothers, part of the group trying to build the house, were first thrashed by the rivals and then forced to drink acid carried by members of the second group. The assailants, then, fled from the scene.
An FIR has been registered at the Sitamarhi town police station on the written complaint of the family members of the injured, later identified as Ram Sevak Rai, Shankar Kumar and Ramesh Kumar, all brothers.
Sitamarhi sadar deputy superintendent of police (DSP) Sanjay Kumar confirmed the incident.
"We are conducting raids to catch the culprits, who are absconding from the village since the incident," he said.
Six persons, including Vijay Rai, Badri Rai, Upendra Rai and Sanjay Rai, were named accused in the FIR, the DSP added.