His victims were all aged under 12, and the offending spanned more than two years.
The youngest victim was just 6 years old when the man offended against her, making her expose herself to him while he took photos of her genitals and touched her, the summary of facts said.
After taking the pictures he showed them to the child, who was embarrassed and asked him to delete them.
A few months after the incident, which happened last year, the girl completed the Keeping Ourselves Safe Programme at school and realised what the man had done to her was wrong.
Police were notified and a search warrant was executed at the man's home. His cellphone was seized and the photographs of the girl were discovered.
The second victim, another niece, was aged between 9 and 11 years old when he abused her, the summary said.
He sexually assaulted her on more than one occasion and asked her to perform an indecent act on him, which she refused.
On another occasion he drove the girl and her cousin to a nearby football club and forced the girl to go under the bleachers with him, assaulting her as she tried to wriggle underneath the bleacher in the hope he could not fit underneath and reach her.
When the girl's male cousin spotted them and said he wanted to leave, the man told the girl "you're lucky" before they all got back in the car and left.
On another occasion he made the girl expose herself and took photos of her on his phone.
The third victim, who was the girl's cousin from the earlier incident and was also the man's stepson, was 8 years old when he was abused.
The man made the child perform an indecent act on him, and filmed the abuse. While the man's face cannot be seen in the video, his voice can be recognised as he gives the child instructions.
He also made the boy expose himself and took photos of his genitals.
Judge Arthur Tompkins said more than 2800 other images of child exploitation were found in the man's possession.
In court today the man's lawyer, Catherine Gisler, argued her client should be entitled to discounts to his sentence for "genuine remorse" and previous good character.
But Judge Tompkins did not believe there was evidence of remorse and said the man's lack of previous convictions was undermined by the substantial collection of child abuse images, and the length of time the offending occurred over.
He noted the pre-sentence report contained a "number of unfortunate comments which do not reflect well" on the man, which appeared to minimise his offending and show he had not internalised responsibility for it.
He said the man repeatedly referred to the offending as "something that happened" rather than something he had done, appeared to attribute some of it to his use of cannabis, and "endeavoured to minimise the effect on one of the victims by suggesting that an apology after the offending might have absolved or reduced his responsibility".
Gisler said some of her client's comments to the report writer had been "misinterpreted" and that the man accepted responsibility, regretted the offending, and was "abundantly aware" of the impact it would have on the children.
Judge Tompkins allowed the standard discount of 25 per cent for the man's early guilty pleas, but did not provide any other discounts.
He sentenced the man to nine years in prison and ordered the destruction of the objectionable material. A first strike warning has already been given, and the man has automatically been added to the child sex offender register.
