An off-duty police officer was believed to have shot and killed her boyfriend and their 1-year-old son, and then turned the gun on herself in an apparent murder-suicide at their Brooklyn home on Monday morning, investigators said. The officer’s 19-year-old son managed to escape the three-story house on East 56th Street by fleeing through a bedroom window after hearing the sound of gunshots, said Paul J. Browne, the Police Department’s chief spokesman.