The boys, ages 15 and 14, and a 16-year-old girl allegedly posted death threats on Facebook aimed at students and staff at their school.
The three were arrested Wednesday. They are students at Le Sommet, a secondary school in the Charlesbourg borough of Quebec City.
Quebec City police Const. Catherine Viel said investigators believed the students intended to act on their threats and had set a date.
School, lockers searched
Viel said the lockers of the three students were searched on Wednesday, but no firearms were found.
In light of the Newtown, Conn., school shooting, Le Sommet principal Yves Savard said he alerted police immediately after hearing about the alleged threats.
"If I called the police, it's because I judged that it was a serious situation and I did not want to take any chances. Otherwise, we do not know what could have happened," said Savard.
He said police conducted a search of the school premises Wednesday night but didn't find anything.
The school was open Thursday.
In the morning, the principal spoke with students and staff about the situation. A letter was also sent home to parents.
The three suspects have been interrogated by police and are currently being held in a youth detention centre.
They are scheduled to appear in court again on Monday for a bail hearing.