Jose Andrade, 29, will be recommended for deportation after serving his sentence.
He admitted the three offences, which occurred in the late summer.
Crown Advocate Fiona Russell told the court that the party had taken place at a flat, and Andrade had arrived some time after it started.
Victim B, a 16-year-old girl, was dancing when Andrade arrived. He smacked her on the bottom, to which she responded by slapping his face and telling him not to touch her.
He said he was joking.
Soon after he took some of her property. She took it back and went to place it in a bedroom.
Andrade followed her in and grabbed her.
She told him she was 16, to which he replied: ‘I’d have five minutes with you and do time for it’.
He threw her onto the bed and lay on top of her, but moved off when she hit his face.
The girl left the room and spoke to the tenant of the flat, who made Andrade apologise to her, which he did, again saying it was a joke.
Later, victim C, an 18-year-old man, was coming out of the bathroom, where victim B and others had gathered.
Andrade asked him if he wanted to see his genitals, and, when victim C declined, Andrade exposed himself.
Andrade asked victim C to expose himself but victim C refused. Andrade slapped victim C’s groin and walked off.
When victim A, a 20-year-old woman, arrived, she saw a friend of hers and Andrade in the bathroom, and went to join them.
She knew Andrade already and he said he wanted to talk to her privately. He made the other people leave the bathroom and locked the door.
Then he sexually assaulted her.
This had hurt her and she screamed which led to others forcing the door open. They removed Andrade from the room and the party.
As he left he spoke to the woman saying: ‘I will find you and kill you.’
A subsequent medical examination of victim A found internal and external minor injuries, including bruising around her neck and thighs.
During interviews with police Andrade said he and victim A had kissed consensually and it was she who had locked the bathroom door.
He said the other girl was also lying and he could not understand why they were setting him up.
Victim impact statements had been made by the complaints which showed the long-term effects of his actions, said Crown Advocate Russell.
For Andrade, Advocate Liam Roffey said his client had never appeared in court before.
That night he had drunk so much that he could not remember much about what had happened.
He had also taken five or six tablets of Ecstasy.
Victim A had worked with Andrade and said that night he was not himself.
Delivering the sentence, Judge Russell Finch said the court was horrified by what had taken place between Andrade and victim A. The assault on victim B was doubly disturbing because of her age. The assault on victim C was less serious and less protracted.
‘You are a violent repeat sexual predator who inflicted significant violence on two of the victims and an unpleasant assault on victim C,’ said Judge Finch.
Andrade was sentence to three years six months for the assault on victim A, two years for the assault on victim B and 12 months for the assault on victim C.
The Lt-Governor would be asked for permission to deport him upon his release, but, should he remain in the island, he would be subject to a sexual offences notification order for 10 years and an extended sentence of three years.
‘We believe that it is in the public interest that you do not remain here,’ said Judge Finch.