3 Mother Letters
In Hebrew the three mother letters are Aleph, Mem, Shin. These are related to the 7 double letters and the 12 simple letters. Together these make up the 22 letters of the Hebrew Alphabet which have a mystical and esoteric meaning particularly through the cabala.
Aleph Mim Shin
Air Water Fire
Ca Ba La
On the caduceus they are arranged in a vertical order and linked with 3 colours: Aleph Mem Shin Yellow Blue Red For links with days of week, month, year: http://www.inner.org/hebleter/letterof.htm