The trio, aged 16 to 17, appeared in a children's court on Wednesday where they each pleaded not guilty to the murder of Aidan Becker, 22.
Mr Becker, an off-duty security guard, was fatally stabbed at Mernda train station on Melbourne's northern outskirts on March 6.
It's alleged the group of teens pursued and attacked Mr Becker as he tried to help a 14-year-old boy who was being robbed.
The three teens and co-accused Mike Pikos, 18, were each arrested and charged with murder and armed robbery.
The teens on Wednesday were committed to stand trial in the Victorian Supreme Court after entering pleas of not guilty.
They were remanded in custody to face a directions hearing on August 26.
Pikos will face a committal mention hearing in Melbourne Magistrates Court on August 19.
A vigil was held for Mr Becker in the days after his death, with hundreds gathering in Mernda to remember the 22-year-old as a young man who was devoted to protecting others.
"My beautiful boy would have done it all again in a heartbeat because that's the boy he was," Mr Becker's mother Natalie told the crowd on March 13.Â
The parents of the schoolboy Mr Becker was protecting also paid tribute, saying he showed extraordinary courage and compassion to save their son.
Dozens of bouquets and several handwritten notes were left near the site of the alleged crime.
His high school Assumption College described him as mature, humorous and kind-hearted.
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