The third dimension, as it implies, relates to three dimensional space
or volume. As a progression from the flat 'two' universe, the introduction
of depth allows for the creation of the infinity of forms, textures, molecules
and characteristics which comprise our three dimensional experience of
the world. Density, Acoustics, Velocity, Thermodynamics, Minerals, Transparency,
Explosion, Projection, Crystallization are a few of the essential features
of three dimensional reality.
It should be stressed that none of these 'dimensions' exist independently of one another but are layers of complexity which self organize in our experience of life. For example, the existence of a particular mineral crystal, although reducible to a three dimensional description, is created in the context of the fourth dimension of 'time', just like this text is more than just a two dimensional field of pixels.
Three corresponds to the Empress archetype, the weak force, the element air, the Holy Trinity, and the process of creation - preservation - decay which is identified in Hindu mythology.