"A dude just came up with, like, blood all over him, asking for help," a McDonald's employee said in one of two triple-0 calls played to an ACT Supreme Court jury on Monday.

A 17-year-old boy accused of murder is on trial in that court, having denied stabbing an 18-year-old man to death during what has been called a "quick and confused melee" at the Weston Creek skatepark in September 2020.

The boy has, however, admitted recklessly inflicting grievous bodily harm on the deceased's 16-year-old cousin by knifing him in the back during the same two-minute melee, which involved 12 people.

Neither the accused, who was 15 at the time in question, nor the cousins can be named for legal reasons.

On Monday, the third day of the murder trial, the jury heard evidence from 10 people who were not present at the scene but who were connected in some way to the events of the night in question.

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Many had attended gatherings at three locations in the Canberra region, from which groups of combatants had converged on the skatepark.

The brawlers travelled there in response to what one witness called a "pretty pointless" Snapchat argument, which culminated in the surviving cousin and another boy agreeing to have a physical fight.

The jury was on Monday shown a number of snippets of the social media stoush, the details of which are suppressed.

One witness, who identified himself as the deceased teenager's "best mate", described how the 18-year-old seemed fine when he arrived at a "boys' night" to pick up his cousin and drive him to the fight.

"He was just looking out for a little cousin that just seemed stressed," the witness said of the deceased.

"He just wanted to ... make sure that he was there to be by his [cousin's] side."

The jury has previously been told the cousins arrived at the skatepark to what might be described as "an ambush", during which the elder of the pair was stabbed six times and killed. The younger of the relatives was seriously injured when the accused knifed him once in the lower back.

Another of Monday's witnesses described to the court how he had questioned the alleged killer in a car hours after the incident, saying he remembered the accused "not really wanting to talk about" the fight.

"I remember him being very standoffish about it and not giving a straight answer," this witness said.

Under cross-examination by defence barrister David Barrow, who described what happened in the car as an "interrogation", the witness denied intentionally misleading police by failing to mention it in an interview with investigators. The witness apologised for this, saying he was "nervous and stuff".

Monday's evidence concluded with the McDonald's worker's evidence and the triple-0 calls being played.

The trial, which is expected to run for several weeks, continues on Tuesday.

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