Trevor Wright, 67, of Ballingdon Street, Sudbury, was found guilty at Ipswich Crown Court of five charges of rape and seven of sexual assault.
He had already admitted six more sex assaults on the same three victims.
Judge Martyn Levett granted Wright conditional bail and told him he would be sentenced later this month.
One rape took place in a car belonging to a customer at Wright's garage in Ballingdon Street, the court heard.
Other offences took place at a stables where Wright kept a horse.
One sex assault was in a swimming pool in Devon while Wright was on holiday, prosecutor Andrew Thompson said.
Wright said although he was "sex obsessed", he denied the further assaults or committing rape.
He had told police he had been a heavy drinker and suffered from memory problems.
A garage owner who preyed on three schoolgirls between 1977 and 1989 has been convicted of rape and sexual assault.