SINGAPORE - Three teenagers, aged between 16 and 19, were on Thursday convicted of grievously hurting a 39-year-old man, who ended up being bedridden with brain damage.

Muhammad Syazwandi Abdullah, 16, Mohammad Zaidi Zahad, 17, and Mohammad Noor Helmi Mohammed Herman, 19, pleaded guilty to the offence, committed earlier this year.

The court heard that at about 3.30am on March 6, the trio, together with another friend, Ahmad Sholihin Mosa, 23, went to the 7-Eleven store at Block 201E Tampines Street 23.

Outside the store, 39-year-old See Chien Hwa supposedly slapped Helmi on his back before walking away.

Then, Mr See went to sit on a nearby bench with his friend, and shouted loudly at the group.

Unhappy, Helmi consumed 10 nitrazepam tablets, walked around the area and picked up an empty glass bottle that he found. Nitrazepam, a controlled drug, is used to treat anxiety and insomnia.

He asked his three friends if they wanted to assault the victim, and all agreed .

Shortly after, the group of four went up to Mr See.

Helmi swung the glass bottle at Mr See's head, shattering it into pieces, before kicking the latter's upper body, causing him to collapse to the ground.

Syazwandi, Zaidi and Sholihin then assaulted the man. Mr See was kicked repeatedly and punched on his face, abdomen, groin and knee.

The four friends then left him bleeding on the ground. But Helmi returned shortly after and continued to punch the victim repeatedly.

A passer-by later saw him on the ground, covered in blood with glass fragments around him, and called the police.

The court was told that Mr See suffered brain damage and, till today remains unable to respond to verbal instructions. He is incapable of communicating verbally and physically.

A medical report in April stated that doctors are unable to guarantee his recovery and will need to re-evaluate him later to assess his mental capacity.

He was discharged from hospital last month, and has since been at a community hospital.

While he noted the ages of the three convicted teenagers, Deputy Public Prosecutor Jason Chua said a sentence of probation would be "wholly inappropriate" given the severity of the offence they committed.

He asked for Syazwandi and Zaidi, who also pleaded guilty to a charge of theft, to be sent to the Reformative Training Centre (RTC). DPP Chua also said the prosecution intends to press for "a severe custodial sentence, with caning" for Helmi.

Describing the assault as "particularly vicious and heinous", District Judge Mathew Joseph said it was one of the worst that he hadheard.

He called for a RTC suitability report for Zaidi and Syazwandi, who will be sentenced in three weeks' time.

Helmi, meanwhile, will be sentenced in two weeks' time. He also pleaded guilty to charges of drug consumption, theft, committing a rash act, criminal intimidation and arming himself with a dangerous weapon. In September last year, he pointed a knife with a 25cm long blade at a 29-year-old man, whom he accused of staring at him at the void deck of a HDB block.

Sholihin's case will be heard in court on Friday.

The maximum penalty for causing grievous hurt is up to 10 years' jail, with fine and caning.