A vegetation fire spread over three rugged acres of grassy oak woodland above Deer Park School in Fairfax just before 5 p.m. on Saturday. The fire was contained within three hours, with the help of three departments, 89 firefighters, four water-dropping aircraft and a spotter plane. “We don’t know exactly how it started,” said Marin County Fire Department Battalion Chief Mike Giannini. “The fire marshal did find fireworks near the origin, so it’s suspected that…