A County Court judge, who cannot be named because it could identify the teenager, had been due to sentence the boy on Tuesday but agreed to adjourn the case for six weeks to give the Department of Human Services time to compile a report on the boy's suitability to serve his jail term in a youth justice centre.

After the judge sentences the boy in December, it will be up to the Adult Parole Board to decide where he serves his sentence.

The teenager was 17 when he randomly attacked three women on three separate occasions over a three-month period from November last year to January this year.

He choked each of his victims until they lost consciousness before he sexually assaulted them during the brazen and predatory attacks.

The teenager, who turned 18 in August, pleaded guilty to a number of charges including rape, attempted rape, robbery, assault with intent to rape and indecent assault.

The Children's Court decided to send the case to the County Court because the offences were so serious. If the teenager had been sentenced in the Children's Court, the maximum sentence he could have received was three years' jail.

The teenager's lawyer told an earlier court hearing that the boy's childhood had lacked stability because of the social stigma suffered by his mother in Bogota after she had given birth to the boy out of wedlock.

The teenager was moved from house to house and spent seven years living with his paternal grandmother and uncle. His father had been jailed for drug offences when the boy was four years old.

At the age of 14, the boy moved to Spain with his mother but had problems at school and returned to Bogota a year later.

He began associating with street gangs before emigrating to Australia in early 2013 to be with his mother, who had moved to Melbourne to start a new life.

The teenager's first victim was a 22-year-old woman who was walking along Bourke Street at about 12.30am on November 9 when she was grabbed and forced into a laneway before being attacked.

The second victim was a 24-year-old woman who had been talking on her mobile phone outside a car dealership on Kings Way, near Albert Road, when the teenager grabbed her from behind between 2.30am and 3am on December 7 and sexually assaulted her.

The third victim was a 22-year-old woman who was grabbed in a headlock by the teenager as she was walking in Flinders Lane about 6am on January 18.

The teenager put a shard of glass to the terrified woman's neck and forced her to walk with him through the city for more than a kilometre before he raped her twice in a stairway near th Southern Cross railway station.

The teenager is expected to be deported to Colombia when released on parole but his mother has indicated she will stay in Melbourne with her new partner.