The fire started around 4 a.m. Saturday on the 800 block of DeKalb Street.
Upon arrival, firefighters say flames were shooting from the twin home, and several people were trapped inside.
"The fire station's located about a block away from the scene. Firefighters arrived and found a three-story dwelling, with heavy fire visible from the first floor of the home," said Norristown Fire Department Chief Tom O'Donnell.
Marco Dominguez, who lives across the street, says he woke up when he heard his neighbor screaming for help.
"He was in shock because his two daughters were in there," said Dominguez.
Soon after, Dominguez says he saw firefighters pull the two girls out of the home and start CPR.
"They wouldn't respond so they took them to the ambulance, and took them to the hospital," said Dominguez.
Officials say three people were killed - two adults and one child.
Six others were injured, including a child who was taken to Children's Hospital of Philadelphia in critical condition.
So far, no word on the conditions of the five other victims.
The landlord of the 827 side says the damage there is mostly from smoke, and she hopes her tenants can return in a few days. But the 825 side, where the fire started, has extensive damage, and the chief says it will take quite some time to rebuild that property.
Seventeen people are being helped by the American Red Cross.
Norristown firefighters went door to door, handing out smoke alarms and information.
Code enforcement officials investigated alongside the Fire Marshal and State Police.
While it appears the home was divided into apartments, the chief says the property where the fire started is only licensed as a single-family home.
"This is an example of one owner who may have been doing something behind the scenes that we may not have been able to catch," said Norristown Councilman Hakim Jones.
Officials are investigating the cause of the blaze.