Detectives are investigating possible links between the assault and a series of sexual attacks against men in the capital in recent months.
In the latest incident, a 19-year- old man was raped on Hampstead Heath in north London on Tuesday after being forced off a Northern Line train. Police appealed for witnesses to the abduction, which took place at 6.30pm.
The attack followed six previous assaults on men in London during the summer months. One took place on an Underground train; in another, a man was raped after being abducted outside Leicester Square station. In August, a man was raped in lavatories at another central London tube station.
In the latest case, the victim was travelling in an empty front carriage to Golders Green, where he had arranged to meet his girlfriend at a night school.
The three men, armed with a handgun and a knife, walked in through a connecting door while the train was between stations. They forced him on to another train to Hampstead, where he was taken to the heath and assaulted by all three.
Police believe the area was busy at the time of the attack, which lasted five minutes. They said the victim was in shock.
The string of attacks in London has begun to focus public attention on the subject of male rape, sometimes termed the last taboo. While attitudes to female rape have been transformed over the past 20 years, there is a reluctance to acknowledge a phenomenon that challenges men's fundamental notions about their masculinity and invulnerability.
Male rape does not exist as a criminal offence under British law; the assaults are recorded as non-consensual buggery. The Metropolitan Police investigated 30 such offences against adult males in the year ending last April, but welfare organisations believe that as many as nine in 10 incidents are not reported.
Survivors, a national counselling organisation for male rape victims, received 600 calls last year. Henry Leak, chairman of the organisation, believes that they represent the tip of the iceberg.
Mr Leak said that reluctance to report attacks stemmed from feelings of shame and humiliation. 'There is the fear that he will not be believed or that people will think that he asked for it, that he must be gay, or weak, unable to defend himself.'
He also pointed to differences in the law - convicted rapists can be imprisoned for life, whereas buggery carries a maximum sentence of 10 years. The perception by some people of police officers as homophobic also plays a part.
However, the growing seriousness with which this offence is treated is reflected in the Metropolitan Police's piloting of a victim support unit.
According to Mr Leak, male rape is not confined to the homosexual community and, like female rape, it has more to do with power than sexuality. The effects, he said, can be devastating and long- term, with feelings of frustration, powerlessness and anger experienced by victims.