C. Lalramzauva (39) murdered his mother Lianngengi (76), his eldest brother C. Vanlalbela (50) and Vanlalbela's son Chhakchhuakthanga (11) on September 2, 2013 inside their home in Tawipui North.
The prosecution said that Lalramzauva, after committing the crime, ransacked the house to mislead the police into believing that it was an outside job.
He left the crime scene to stay with his widowed lover and pretended to learn about the incident the following morning, the prosecution said.
He, however, later confessed to the crime and said that he murdered all his family members as they objected to his marrying his lover.
Judge Marli Vankung convicted and sentenced him on Friday.