Kevin Waterston (30) and Philip Christie (27), both c/o Perth Prison, had already admitted their role in the crime.

All three will now be sentenced at the High Court in Glasgow on July 1.

They presented a knife or similar instrument to an employee of 3000 RPM, Westport, Dundee, on December 12, threatened to repeatedly stab him and stole approximately £2000, a pair of laptop computers, a mobile phone and wallet containing £30 along with other cards and papers.

Jackson, also c/o Perth Prison, had pleaded guilty to a charge of being concerned in the supply of heroin on the same day.

Advocate depute John Scullion today told the High Court in Perth that Jackson was caught with just over 15 grams of heroin with a potential street value of £1500.

The court was also told that Waterston, Christie and Jackson had amassed more than 120 previous conviction between them.

All three have numerous convictions for crimes of dishonesty and Waterston and Christie have also previously been convicted for possession of a knife.

The shop worker had previously told the court of his terror on the day of the raid.

All three robbers were masked and armed with knives when they burst into the shop.

The man described the incident as like “something out of Hollywood”.

“I was so shocked I just went into survival mode,” he said. “I just wanted to get out of there with my life.”

The victim, who was the sole employee in the shop at the time, said he saw “three gentlemen jumping the counter and heading straight for me”.

“My initial impression was that it was a joke, but I quickly realised this was not the case when I saw their knives. I was pushed to the floor in the rear of the shop as the three men surrounded me with knives drawn,” he said.

The man said one knife, which he described as being six to eight inches long, had been held at his gut and the other two blades at his throat.

“At this point one of the men told me, ‘If you don’t give us the money right now I will put four or five holes in you’,” he said.

“We all know about knife crime in this country. I didn’t want to be one of those people who end up dead. I offered to open the till and just asked them not to hurt me. When people threaten you with knives you just give them the money. It is not worth losing your life for.”

Lord Malcolm told Waterson, Christie and Jackson they would be sentenced on July 1 following the preparation of reports which would assess the risk they posed to the public.