The accused pleaded guilty to three charges of premeditated murder and three of conspiracy to commit murder.
During the youth's appearance, the judge allowed a 911 call made by one of the victims to be played in court.
The victims were sisters aged 22 and 17 as well as a 17-year-old boyfriend.
The teen, who was 16 at the time of the Feb. 11 killings, also allegedly conspired to murder the mother of two of the victims.
Sentencing arguments are to be heard in October.
Trois-Rivieres police say they intercepted two youths moments after they had allegedly emerged from a home in the community.
Authorities say jealousy was considered to be a likely factor in the deaths.
The other accused, who is 17, will face trial by judge and jury on similar charges. He is due to return to court on Wednesday.