Brandon Menard called 911 to report the deaths, but was later arrested.
He faces life in prison without parole when he's sentenced next month.
David and Toni Menard were found stabbed to death inside their home on the 17000 block of Stare Street.
Menard's 16-year-old sister was found stabbed multiple times in the back yard.
The motive for the killings has never been made clear.