Family and friends of three people killed in a frenzied stabbing attack in mid-north South Australia have spoken of their hatred and anger toward the murderer.

Teenager Chantelle Rowe and her parents Andrew and Rose were killed at the family's home in Kapunda in November 2010.

Jason Alexander Downie, 20, has confirmed his guilty pleas to three counts of murder in the Supreme Court in Adelaide.

Downie listened as the Director of Public Prosecutions (DPP), Stephen Pallaras, described the injuries suffered by each of the victims.

He said 16-year-old Chantelle was stabbed at least 33 times, her father at least 29 times and her mother at least 50 times.

Mr Pallaras described the grisly scene in the Harriet Street home.

"It is no exaggeration to say the hallway and floors throughout the house were covered in blood," he said.

"It appears from the position of the bodies of Andrew and Rose Rowe and the positioning of their blood that, even after he had stabbed them and attempted to clean up their blood, he went back to them and stabbed them again."

Mr Pallaras told the court Downie gave a number of contradictory statements to police and lied about cuts on his hands and forearms.

Many of the family and friends of the Rowes who had packed the courtroom left as the extent of the crime was revealed.

Infatuation

Defence lawyer Greg Mead QC told the hearing Downie was jealous Chantelle Rowe was dating his friend.

He said Downie's infatuation with the teenager had turned to resentment and anger and ultimately homicidal rage.

Guilty plea: Jason Alexander Downie

Mr Mead said many would think his "weedy" client (at 52 kilograms) would not be capable of a such a crime.

"He unleashed an attack which will not seem possible from someone of his stature," he said.

"Obviously he went completely beserk that night. He seems to have lost all control.

"[He was an] immature young man who could not have been aware that he was capable of such violence."

Mr Mead told the court a sexual attack on Chantelle Rowe happened shortly after the stabbing, and family members in the courtroom yelled out "gutless bastard".

The defence lawyer said Downie originally told police "stupid, childish lies" but that had "simply emphasised his lack of maturity".

He told the court Downie eventually cooperated with police, taking them to the site where he had dumped the clothing and shoes he wore during the attack.

Mr Mead said Downie also took police where he said the murder weapons had been dumped, but no weapon was found.

Apology letter

He told the hearing Downie could not remember much of the attack, but was beginning to understand the enormity of his crime and deserved credit for his admission of guilt.

"He has written a letter of apology to the family of the victims but admits no words will restore the Rowes to their family," the court was told.

Mr Mead told the court Downie's own family had been shocked by the nature of the crimes.

"His family must've found it unbelievable that this skinny young man, who never answered back to his mother or been in a fight, had committed this crime," he said.

Mr Mead said his client experienced a mental "numbing" after the attacks.

"It was a self-protection reflex action he had to get past in order to fully face up to what he has done," he said.

Downie sat stony-faced while more than 20 victim impact statements were read to the court.

Surviving son and brother, Christopher Rowe, told Downie he was struggling to deal with his grief at losing his family.

He said he felt empty as he thought daily about those he had lost.

Others told the court of their hate and anger towards Downie and questioned why and how he could commit what they described as an evil and heinous crime.

Downie confirmed his guilty pleas at the latest hearing after questions were raised by prosecutors last month about the legitimacy of his admissions.

Downie will automatically be jailed for life and Justice John Sulan will set a non-parole term next month.

The DPP said Downie was entitled to credit for his guilty pleas but the non-parole period needed to be significant.