alef - mother over six of the aspirated variants of the double letters

mem - mother over six of the muted variants of the double letters and other stops (alef, eyin, tet, qof)

shin - mother over the sibilant letters (zeyin, samek, tsadi, and shin)

There are eight letters remaining:

Three liquids: yod, resh, lamed

Two nasals: mem, nun

Two more gutturals: heh, khet

And an approximant: vav