Investigators said Tuesday the fire began with something cooking on a stove.
The fire broke out at approximately 6:27 a.m. Wednesday on the 900 block of East 15th Street.
Moses McKnight, 4, his 6-year-old sister Bella McKnight and their cousin, 9-year-old Amajhay Ferguson, all died in the fire.
There were six children in the house at the time of the blaze. Three of them live in the home and the other three were cousins staying for a visit.
The father of the kids, Timothy Ferguson, managed to get his wife and three children out safely.
Five firefighters were also hurt in the blaze, which happened two blocks from the Widener University campus.