Le Ngoc Tuan, 41, was fixing a motorbike at the garage in Vinh Loc District of Thanh Hoa Province at around 11:30 p.m. as he detected fire from an area where an electric motorbike was being charged.
He tried to put out the fire using a portable fire extinguisher but to no avail. At that time, his wife and two children were sleeping on the second floor of the house, which has three floors.
Tuan rushed out of the house on fire and screamed for help.
He told police later that he was in a panic state and "forgot" to wake his wife and children to save them.
As the garage has many inflammable materials and substances, the fire grew quickly and by the time firefighters arrived, it had covered the house's first floor.
After around one hour, the fire was extinghuished and police found Trinh Thi Lan, 40, and two boys aged 11 and six died of suffocation in their bedroom on the second floor.
The house has an emergency exit leading straight to its balcony, which is not fenced.
Police are investigating the case further.