Authorities say the victims had gunshot wounds to the upper body. A coroner will release their official cause of death.
Fire crews responded to the 500 block of N. Dalton Avenue at 2:43 p.m. Thursday after getting reports of a large plume of smoke coming from the garage area of the property. Neighbor Francisco Garcia said they heard popping sounds coming from the garage.
"And then the flames were just shooting up out of the garage. We weren't sure what was going on," Garcia said.
The three victims were found inside the residence, as well as two deceased dogs and two deceased cats. The victims' names have not yet been released. Azusa Police Lt. Mike Bertelsen said the victims were one male and two females, all middle-aged adults.
Investigators discovered rifles, handguns, thousands of rounds of ammunition and several incendiary devices inside the home.
The cause of the fire is under investigation.