The three fled the spot after the incident yesterday and are on the run, Gumanpura police station cop Hansraj said.
An argument had broken out last evening between the victim, Devilal Prajapat, and his 50-year-old son over erecting a pillar to lay a water pipeline over the roof of his house.
The son and his wife then dragged the elderly man out of his residence in Kotari-Goverdhanpur area here and thrashed him, the cop said.
Mr Prajapat was then attacked with a sword by his 30-year-old grandson while the couple held him by his arms, the police officer added.
After a complaint by the younger son of the deceased, police have lodged a case under relevant sections of the IPC against the three.
The man's body was handed over to his family members today after a postmortem examination and a hunt is on for the accused, police said.