The shocking claim was made during the reopened inquest into the murders of Karen Edwards, 23, Tim Thomson, 31, and Gordon Twaddle, 21, in the Queensland outback in 1978.
The three friends were on a motorbike adventure, on their way home to Melbourne for the Christmas holidays.
The adventure ended in horrific violence and the trio's decomposing bodies were discovered in bushland at Spear Creek, 12km north of Mount Isa, in October 1978.
All three had been shot in the head.
For almost five decades, the crime has remained unsolved, but a new probe by Queensland coroner David O'Connell hopes to finally solve the mystery.
Part of the reopened inquest has focused on the former accused Bruce John Preston.
He has been linked to the investigation from almost the beginning after being caught in possession of Mr Thomson's missing red BMW motorbike a month after the murders.
Mr Preston initially claimed he bought the bike secondhand, but later pleaded guilty to stealing the BMW and copped a $300 fine.
Cold case detectives charged Mr Preston with three counts of murder in 2019, only to drop the case in 2023.
His older brother on Tuesday told the court Mr Preston was a member of the Black Uhlans outlaw bikie gang at the time of the murders.
"I recall his nickname in those circles was Pedro and reading that name on his jacket," Ian Preston said.
As questioning continued, Ian Preston was threatened with perjury before being granted indemnity against future prosecution over fears his brother was involved in the murders.
"I was concerned that such a thing had happened initially, and in some way, shape or form, that my brother may have been implicated in it, by virtue of the fact that what went on in Mount Isa at the time," he said.
He said he had discussed the case with his family but denied evidence that his father, who has since died, admitted that Bruce Preston was the killer.
"The little bastard done it, but he's a tough little bastard - they can't break him," the inquest was told father Arthur Preston had claimed.
Ian Preston said that while his father was a hard man, and it was language he would have used, he denied first-hand knowledge of the admission.
He conceded the entire family were hunters and very familiar with guns, describing his brother as a "crack" shot.
Despite being in ill health, the court has ordered Bruce Preston to appear in person to give evidence on Tuesday.
The last known sighting of the three friends was at the Moondarra Caravan Park in Mount Isa on October 5 in the company of a man driving a brown and white Toyota LandCruiser.
Post-mortems revealed all three had been executed with a bullet to the head fired from a .22 calibre weapon.
The hearing has accepted the cause of death findings that Ms Edwards, Mr Thomson and Mr Twaddle had been shot in October 1978.
The inquest set down for nine days will hear from up to 40 witnesses as it traces the trio's outback journey and explores events at the Lake Moondarra caravan park.
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