Clarkson detectives Senior Sergeant Gary Saunders said three people entered the store on Marmion Avenue about 9.20pm on July 25.
They moved to a staff-only area of the store and alegedly stole alcohol, Sgt Saunders said.
He said they were seen by an employee and challenged and asked to leave the store.
Security staff attempted to recover the property they were carrying.
One of the acused allegedly armed himself with a broken stake as a large, sharp-pointed weapon, threatening staff members and threatening to kill an employee.
Sgt Saunders said staff retreated out of concern for their welfare and the accused fled the scene, but during the commotion, another customer apparently saw an opportunity to allegedly steal alcohol.
He said they charged that man, an 18-year-old from Butler, with stealing as a separate offence to the robbery.
According to Sgt Saunders, detectives charged a 15-year-old Quinns Rocks boy who allegedly used the stake during the incident with aggravated armed robbery, and he will appear in Joondalup Children’s Court on August 24.
Another 15-year-old boy was charged with aggravated robbery, and will appear in Perth Children’s Court on August 20.
Sgt Saunders said they had charged a third boy, a 16-year-old from Ridgewood, with aggravated burglary and stealing, and also apprehended him on a return to prison warrant.
He said the accused teenager was due to appear in Joondalup Children’s Court today.