"All these acts were premeditated in my view. They were repetitive acts," he said.
"Mr Samson-Witana, your offending smacks of a total disregard for the impact of your acts upon your victims."
The court heard the teenager met the first two girls, separately, in November last year at his Balclutha home.
After drinking alcohol, he invited the first to watch a movie with him in the lounge.
While they sat on the couch, Samson-Witana pulled a blanket over the two of them.
Soon after, he violated the teenager.
"The victim did not want to say anything because she was concerned it would cause dramas," a police summary said.
He committed similar acts against the second victim before police intervened.
"The charge was laid, you appeared in court, conditions were placed upon you, one particularly being not to associate with any person under 16," Judge Phillips said.
"That did not concern you at all."
While on bail at another Balclutha house, Samson-Witana met the second victim again and they again had sex.
Despite his indiscretions, the defendant was bailed again.
In May, a third girl visited his home, the pair drank alcohol and then had intercourse three times over the following days.
"You took no steps to find out what her age was. You were reckless in the extreme in that regard," the judge said.
"The victims are entirely vulnerable ... You really used these young girls and you didn't at any stage consider the consequences of your acts upon them."
In an interview before sentencing, Samson-Witana blamed the victims for what happened.
The report on his attitude made for "disturbing reading".
While the girls had not indicated they suffered severe trauma as a result of the sex crimes, the judge said the impact on them might only manifest later in their lives.
Samson-Witana would also feel the effects of his decisions long after he had been released from prison, Judge Phillips said.
"As a result of going to prison, you will, upon release, be registered as a child sex offender and that will be a permanent recording. You'll be on that register for the remainder of your life," he said.
Samson-Witana was jailed for two and a-half years.