The triple murder happened in February 2014.
The 18-year-old man was 17 at the time of the grisly killing of three teenagers inside a house on Sicard Street.
Neither he nor his co-conspirator, who was 16 at the time, nor the three victims — two females and one male all in their teens or early 20s — can be named due to the Youth Criminal Justice Act.
The younger of the two accused pleaded guilty to the same charges a year ago. The older had originally opted to stand trial.
However, he reversed his decision and pleaded guilty at the Trois-Rivières courthouse on Friday, in the presence of the victims' families.
The younger teen is expected back in court Monday for the start of sentencing arguments. The judge said it was possible he would sentenced as an adult.