Paul Drinkwater, now 52, of Maidenhead, Berkshire, carried out a campaign of terror over three weeks in Berkhamsted in November and December 1984.
He was found guilty at St Albans Crown Court of seven charges he denied, including rape and possessing a knife.
New DNA evidence helped secure the conviction.
Judge Marie Catterson remanded him in custody for sentencing on 30 January.
Drinkwater was convicted of the rape and robbery of a 15-year-old girl in Durrants Lane on 25 November and possessing a knife.
He was found guilty of raping an 18-year-old woman and indecently assaulting another 18-year-old woman on 16 December in Bullbeggers Lane.
During the attacks, in which Drinkwater wore a woollen mask, he kept saying: "I hate slags, tarts and lesbians," the court heard.
Prosecutor Ann Evans said: "This is a case about every woman's worst nightmare.
"They are knife-point rapes where the perpetrator wore a balaclava and took young girls off the street late at night."
Mrs Evans said on 14 January this year a DNA profile from Drinkwater was provided to a forensic scientist.
A swab from the 15-year-old victim was matched as "seven million more times likely to come from Paul Drinkwater, rather than a person unrelated to him," she said.
Samples taken from the trousers worn by the 18-year-old rape victim matched Drinkwater.
"The likelihood of this sample coming from someone unrelated to the defendant was put at 1 in 43 million," she said.
Drinkwater lived with his girlfriend in Berkhamsted at the time and had been linked to the crimes since 1984.
"Through advances in DNA analysis, forensic scientists were able to match the DNA from these two crimes to this defendant," Mrs Evans said.
A man has been convicted of raping and sexually abusing three teenagers at knife-point in Hertfordshire 30 years ago.