The term 'Big AGE Era' refers to a period of time in the late 19th and early 20th centuries when the United States experienced rapid economic growth and industrialization. This period was characterized by large-scale business expansion, increased immigration, and technological advances. The three-letter solution for this crossword is AGE, which stands for the AGE of Industrialization. This era saw the rise of large corporations, the development of new technologies, and the growth of cities. It also saw the emergence of labor unions and the rise of the middle class.