The youths' surprise guilty pleas in a courthouse in Wetaskiwin, Alberta, came on the day their trial was scheduled to begin, the Canadian Broadcasting Corp. reported.
The teens were 17, 16 and 13 at the time of the July 11, 2011, shooting that killed Ethan Yellowbird. The boy was shot in the head as he slept in his grandfather's house in Hobbema.
The three defendants, whose names were not released, were arrested in January. They face a maximum of two years in custody and one year of community supervision when they are sentenced Nov. 21, the CBC said.
The Royal Canadian Mounted Police said they believe the three teens have gang affiliations, but prosecutors gave no possible motive for the shooting.