One of the three suffered 80 percent burns to his body.
Witness Jonathan McDonagh told Australian Associated Press he tried to help the injured, taking off his shirt and using it to put out the flames.
"I came driving in and saw three people come running out... They were on fire," McDonagh said.
"I had the radio on and didn't hear the bang, but I saw a puff of smoke.
"They were really burnt badly."
Some of the injured were electrical contractors inside a store at the Galleria centre in Perth, an ambulance spokesman said.
"All of these people are in for a long stint in hospital," the spokesman said.
A woman who suffered smoke inhalation was also taken to hospital while several other people were treated for shock and smoke inhalation at the scene.
Fire and emergency services cordoned off the area with investigations into what caused the blast underway.
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