The victims - aged 3, 8 and 10 years old - were sleeping in their home in the town of Mati in Davao Oriental province, 1,025 kilometres south of Manila, when the fire broke out.
People search among the remnants of a building after the fire.
Their grandmother had left to stay at a neighbour's house while their mother was out working as a massage therapist, said Christian Cena, the town's fire marshal.
The grandmother was able to take two other grandchildren to safety, but when she came back to rescue the three other kids, she could no longer go inside due to thick smoke, Cena said.
Investigators were still determining the cause of the fire, which destroyed at least 25 houses and left more than 130 people homeless, Cena said.
