The blaze began early morning, an unnamed woman witness living nearby on Le Thanh Duy Street, Xuan Moc District, said.
She was sleeping when she heard explosions and smelled something burning, and woke up to see a kiosk at the shop going up in flames.
"I called my husband and others to bring handheld fire extinguishers to put out the fire."
But by then the fire and smoke were too strong, she added.
Dozens of police officers and many firefighter trucks were dispatched to the scene.
Inside the workshop, in the attic, firefighters found the charred bodies of Huynh Tan Duong, 46, his 10-year-old adopted daughter and an unidentified woman.
Three motorbikes and many electronic items were destroyed. The flames also spread to the two-story house belonging to the shop owner, damaging the living room.
The police are investigating the cause of the fire.