His alleged accomplice, a 17-year-old male youth, chose to stand trial. He is facing the same charges and is to appear in court on Wednesday.
Two sisters and their male friend, all of whom were in their teens or early 20s, were killed inside a home on Sicard Street last February.
CBC News cannot name any of the victims nor the suspects due to provisions in the Youth Criminal Justice Act.
During his court appearance on Monday, the 16-year-old was reminded by the judge that he could be sentenced as an adult for the murders, which means a minimum sentence of life in prison with no chance of parole for at least 10 years.