San Diego Hardware Co., long a landmark in that city, was heavily damaged during a three-alarm fire in the Gaslamp Quarter. No injuries were reported. The stubborn, smoky blaze was reported at 4 p.m. and frustrated more than 75 firefighters for almost an hour as they donned masks and air tanks to try to find the fire's origin in the back of the store. The store's owners are in the midst of expanding through the back into an old hotel, which is undergoing remodeling. Investigators were searching the walls between the store and old hotel for clues to the fire's origin. Firefighters worked to keep flames from spreading to paint and other flammable materials.