A fire truck apparently slipped out of gear, crashed into the back of a firehouse and caught fire Saturday, injuring three firefighters, officials said. Firefighters were moving the truck at the station in the city's West Oak Lane section when it lurched forward. Investigators believe the truck's transmission had jumped out of gear, but the exact cause remained under investigation. A second company was brought in to help fight the blaze. The firehouse, used to house reserve fire trucks, was seriously damaged. Two vehicles also were damaged. One firefighter was hit by the truck but was listed in good condition, and two other firefighters were released after treatment for smoke inhalation.