The explosion outside an auto body shop in the city's Feltonville neighborhood caused serious burns to a man and woman and a 13-year-old girl. Police said that nine other victims were taken to area hospitals for observation.
The three seriously injured victims suffered second- and third-degree burns, police said.
The incident occurred at the La Parrillada Chapina food truck around 5:30 p.m.
Jane Kim, the owner of a store next to the auto body shop, said that the food truck is parked outside daily. She said that she heard the explosion and that firefighters told her that a propane tank on the truck exploded.
Neighbor Luis Rivera told The Philadelphia Inquirer that his home across the street shook from the blast.
"I thought it was a car accident - there are usually a lot on this street," he said. "Then I came outside and saw the lunch truck in flames."
A city fire dispatcher said that the incident is still being investigated.
The two adult victims were being treated at Einstein Medical Center and the teen was being transferred to St. Christopher's Hospital for Children, police said.