A 3-year-old boy fell from a window on the top floor of a three-decker yesterday in Lynn, hitting a parked minivan before landing in a driveway, according to fire officials. The boy was conscious and alert after the fall at about 10:45 a.m., and his injuries were not believed to be life-threatening, fire officials said. The boy was transported to North Shore Medical Center in Salem and later transferred to Massachusetts General Hospital for further examination, District Fire Chief Robert Bourgeois said. The child broke through a window screen before falling from the Chestnut Street building, District Fire Chief James MacDonald said. Police are investigating the incident.