Stephen Charters, 51, formerly of Galashiels, began his crimes as a boy and continued into adulthood.
He had denied the offences but was found guilty of five charges of indecent behaviour and two of rape between 1977 and 1997.
A judge told him they were "very serious charges" before jailing him at the High Court in Edinburgh.
Lady Clark of Calton said it was plain that the victims had been very seriously affected by the abuse for many years.
She said she accepted that most of the early offending was committed when Charters was a child himself and the first rape of an 11-year-old happened when he was aged 13 or 14.
However, she said the further rape he carried out on a girl aged between six and eight took place when he was "a young adult".
Lady Clark told him: "I take into account that you are a first offender, that you led a useful and productive life and there has been no offending for many years."
Charters was convicted of sex crimes between 1977 and 1997 in the Borders, Midlothian and South Lanarkshire.
He was convicted of sexual offending from the age of 12 - the youngest age for prosecution in Scotland.
He began by molesting a younger girl at a house in the Borders, exposing himself and carrying out sex acts from when she was nine. He raped her when she was 11 years old.
Charters later abused a second girl at a house in Midlothian and at the Royal Commonwealth Pool in Edinburgh before raping her.
He later made inappropriate sexual remarks to a third girl in East Kilbride and during a car journey.
Lady Clark said: "He is in complete denial about all of this."
She said he did not appear to have "any understanding or recognition" about the effect on his victims.
Defence counsel Shelagh McCall QC told the court that Charters continued to deny involvement in the sex offending.
She said there was a 20-year gap since the last of the offences during which time he had worked for a bus firm and driven children to school.
She said other people had been interviewed and no further allegations were made against him.
Ms McCall added that Charters was prepared to co-operate with offence-related programmes offered in prison.
A man from the Scottish Borders has been jailed for five years for the abuse and rape of young girls.