Gongaihhluni (65) and her grandsons, Alex Ngozasuana (5) and Cindy Lalmuanzovi (2), were burnt to death when they were sleeping after the house caught fire at 1.20 AM, a police official said.
Three other members of the family survived the fire.
One person, who was seen moving around the house suspiciously, was arrested, the official said.
The cause of fire is yet to be ascertained, he said.