The three mother letters, Aleph, Shin and Mem are related with the three main symbols: air, fire and water. So, they are called the three mother letters, and this is because only the feminine aspect has the power to give birth. All of us come from the womb of a woman. That is why they are called the three mother letters. These three mother letters are inside our own particular Monad, Spirit, which is in heaven.
Nun
Shin
The letter Shin ש relates to the trinity, the three primary forces. It is the creative fire at the base of life. It is the fire that enflamed the burning bush before Moses. ש Shin represents the power of creation inherent in every atom, a power that we must harness and direct towards liberation, rather than using our energies to degenerate ourselves further.
Shin
In Kabbalah, the letter ש Shin is the symbol of fire. The divine fire has three wicks as the letter ש Shin has three wicks, which represent the trinity. That trinity is Kether, Chokmah, Binah, Father, Son and Holy Spirit, the First Logos, the Second Logos and the Third Logos.
In Kabbalah, in the Zohar, it is stated that the letter Shin also represents Chesed, Geburah, and Tiphereth [the Monad]. We will say that inside of us we have the letter Shin.