The three accused had set up make-shift stalls on a pavement in the industrial estate of Tuas, earning 4 Singaporean dollars per haircut compared to average hair cut cost of 10 Singaporean dollars in a local barber shop.
Officers from both the Manpower Ministry and the National Environment Agency raided the area and arrested the three yesterday, while others at the scene fled.
The Ministry was questioning them for violating their work permit under the Employment of Foreign Manpower Act.
The Environment Agency was also investigating the three for offering barber services at an "inappropriate site".
More From This Section
Under the Environmental Public Health regulations, it is an offence for an individual to act as a barber in areas such as roads, footways and back lanes.
The three accused said they were moonlighting to earn extra money for themselves or their families.
Indian Das Palash Chandra, 29, said he needed the extra cash as his father fell from a coconut tree and could no longer work as a farmer back home.
"I do this job for my family."
His compatriot Turuvala Viswanadham, 24, said, "I cut hair for extra money to makan (Malay for eating) and drink."
The Ministry had arrested 592 moonlighting foreign workers last year and 567 in 2012. They were either prosecuted, fined or warned.