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.