Between July and August last year. McDermott went to the victim's house and while there raped the young girl.
He then arranged to meet the victim a second time, after asking her to return headphones he had left at her house. They later met at a subway near to her home before going to a field where he raped and sexually assaulted the girl.
The second victim, then 14, was attacked in August 2011 after she had complained to McDermott on Facebook that she couldn't get anyone to buy cigarettes and alcohol for her from a local shop.
He immediately volunteered to help out and got her two bottles of vodka, and a pack of cigarettes. But he also said he expected sex in return.
The girl drank an entire bottle of vodka before going into Agecroft Park with him, where they had sex. McDermott was arrested by police in August last year after complaints against him had been made by one of the girls.
Then, while still on bail for offences against two of the three, he went on the following October to commit offences against his third victim, a 13-year-old girl he again met through Facebook, befriending her and arranging to meet her in a park in Irlams O'Height where he engaged in sexual activity with her.
The court heard McDermott, an only child, was ''less mature emotionally and socially than expected for his age.''
Lawyers said there was no evidence to suggest that he was deliberately trawling the internet seeking likely victims, and his actions were said to be ''opportunistic.''
But after the case Det Con Nicola Wood, of Greater Manchester Police Serious Sexual Offences Unit, said: "Without exaggeration, McDermott is every parent's worst nightmare.
"He trawled the internet and befriended young, vulnerable girls that he went on to rape or sexually assault.
"Some thought he cared about them but all were too young to consent to any sexual activity. A fact he knew full well but chose to ignore for his own sexual gratification.
"This has had a significant impact on all victims and their families and I hope they are reassured that he is now behind bars where he can no longer pose a threat.
"I would also urge parents to be mindful of who their loved ones are interacting with over the internet and to ensure their children are fully aware of the dangers and potential consequences of befriending strangers."
McDermott was also prohibited for an indefinite period from making any contact with the girls. His name will also be passed on to the Independent Safeguarding Authority which can impose restrictions on him.