(Rainier, OR)  --  Authorities are investigating a three-vehicle crash that killed a woman and injured her two children in Columbia County.  Sheriff's deputies say Heather Robison of St. Helens died around 10:30 a.m. yesterday morning on Highway 30 near Rainier.  Her sons, ages six and one, are listed in stable condition at a hospital.  The driver of one of the other cars, 20-year-old Kierstin Merila of Warrenton, is also being hospitalized.