Goh Kim Lian, 56, who had been given preventive detention twice before, pleaded guilty to three out of 11 charges of robbery.

His sentence had been postponed pending a preventive detention report.

Preventive detention, meant for repeat offenders, can last between seven and 20 years, with no chance of early release.

Goh's two accomplices, Poh Liang Huat, 57, unemployed, and Chan Chin Leong, 45, a cleaner, have also pleaded guilty separately and are due to be sentenced on Nov 25.

A district court heard that on July 13, Goh, Poh and Chan entered an optical shop in Balestier Road where Poh claimed he wanted to change his spectacles.

After the supervisor had taken out a frame, Poh whipped out his chopper and grabbed the shop manager, pulling her into the eye-testing room.

While Goh stood watch over the victims, Poh and Chan took a total of $15,000 from the counters. Goh and Chan were each armed with a knife.

Ten days later, the trio robbed a paint shop at MacPherson Road of $15,000.

On July 31, Goh and Poh robbed Mr Lee Tow Hua, 67, of $1,400 at a flat in Circuit Road while the victim and three others were playing mahjong in the master bedroom.