Any lawful building, structure, structure and land in combination or use of such premises existing at the time of enactment of this chapter or any subsequent amendment thereof applying to such building, structure, structure and land in combination or use of such premises may be continued although such building, structure, structure and land in combination or use does not conform to the provisions hereof, except as follows: Any sign which becomes nonconforming upon the date of enactment of this chapter shall be removed or altered so as to conform within three years after such date of enactment.