I can’t read a word of it, but it does remind me that in French the word for “clue” is the same word as “indication” — i.e., it is something which indicates something else. (I think I first encountered this when reading essays about the Arsène Lupin stories.) That seems like a particularly useful bit of alternative etymology in the particular context of the Three Clue Rule (or Three Indication Rule), since the rule can actually be applied widely beyond the format of a mystery.
(For those curious, the English word “clue” derives from “clew”, which originally referred to a ball of thread: Just as Ariadne’s thread led Theseus to the entrance of the labyrinth, so clues will lead you to the solution of the mystery. The example of the labyrinth, I suppose, just indicates another way in which the provenance of the Three Clue Rule can be extended.)