The fire engulfed the rented apartment spanning 20 m2 in the Central Highlands province at around 2 a.m.
Dozens of police officers, firefighters and search and rescue personnel were deployed to the site. As the doors were locked, officers had to bust them open.
They found the body of three people inside, later identified as Nguyen Thi Tuyet Van, 45, her daughter, 15, and Van's boyfriend Nguyen Thanh Ty, 53.
From security camera footage, police confirmed Tran Trong Nien, 52, as the culprit.
He brought a gasoline can to the apartment, poured it all over and set the fire.
After that, he returned to his home around 1 km from the scene and killed himself.
A preliminary investigation revealed that he and Van were in a complicated romantic relationship.