I am told that there is a Third Word ending in -shion apart from cushion and fashion. What is it?
The usual answer is hushion which is a rather unusual Scots dialect word for a footless stocking (a sort of leg-warmer).
A rather better-known Scots word is fushion meaning spirit, energy, gumption or (in referring to food) wholesomeness.
Neither word is in standard use outside Scotland.
The complete Oxford English Dictionary also lists the word parishion meaning the same as parishioner (a member or inhabitant of a parish). However, this word died out in the 16th century without ever settling into a consistent spelling.