The 3 Mother Letters
Sepher Yetzirah, the Book of Formation, says that they represent in the Universe the 3 active elements, in the Year the 3 climates (seasons) and in the Man 3 areas of its body : Alef is the Air, the Temperate Climate, the Chest. Mem is the Water, the Cold Climate, the Belly. Shin is the Fire, the Hot Climate, the Head.
A quick note about the seasons: during the year the 3 seasons, according to Sepher Yetzirah, are alternated in a cycle, Mem (winter), Alef (spring), Shin (summer), Alef (autumn).