Flynn had sexual intercourse with the girl in 2017 when she was 15 and he was 26.
Judge A A Zohrab's decision said Flynn had tricked the girl into thinking they were going to see a relative, and that consequently she had significant trust issues.
"She was scared. She was terrified that it would happen again and she is glad you have been caught out."
The second victim was a friend of the first, who Flynn had messaged sexual images and video to over a span of three years.
The third victim was also a friend of the first, and also received sexual messages including an indecent image from Flynn.
The judge's decision said the third victim had been "disgusted" by the images she had been sent.
"She is scared that someone like you will contact her again on Facebook, and the same would happen, and that has made her wary of meeting new people."
Zohrab took into account Flynn's minimal criminal history and cognitive impairments during sentencing.
"I appreciate that prison is going to be difficult for you, and that you will be vulnerable within a prison environment because of your impairments, so in my view it is appropriate to take that into account by reducing the prison sentence by 15 percent."
Flynn was also entitled to 25 per cent reduction for his early guilty plea.
He was sentenced to three years and three months in prison.
Flynn was denied name suppression.