Long before it was the number of the
Christian Holy Trinity, three was the number of a more fundamental trinity:
Mother,
Father, and
Child. This sort of a trinity arises out of a
polarity.
However, three is also the number of the alchemical principles: salt, sulphur, and mercury. These represent the processes of stasis, change, and dynamic equilibrium. A similar, more modernistic set of three processes is that of stasis, dynamism, and entropy.
The three alchemical principles, when multiplied by the four traditional elements, yield the twelve signs of the zodiac.