The accused pleaded guilty Friday to three charges of premeditated murder and three of conspiracy to commit murder in Trois-Rivieres in February 2014.
He'd originally opted for trial before judge and jury, but entered the guilty pleas before a ycuth court judge.
His co-accused, also a minor at the time of the crimes, pleaded guilty to similar charges last June.
The victims were sisters aged 22 and 17 as well as a 17-year-old boyfriend.
Neither the victims nor the accused can be identified due to a provision in the Youth Criminal Justice Act.
Trois-Rivieres police arrested the two accused just moments after they'd emerged from a residence.
Authorities said at the time that jealousy was considered the likely motive in the killings.
A sentencing hearing for the accused who pleaded guilty in 2014 begins Monday.
(Cogeco Nouvelles/The Canadian Press)