Starting from the first research into elements in Ancient Greece, the five elements earth, water, air, fire and aether were the key components on which everything was based on. In this context, air is usually listed as the third element.
Looking at the development of technology, similar approaches can be found. After roaming the earth and developing seafaring to cross the seas, the air was the third “element” that was explored and conquered by men through aviation.
Third Element hence is a synonym for air, aviation and for the inner drive to conquer this “element” through technology.