You form a superlatives by taking the adjective ἅγιος (holy) and adding ατοτος. I have not yet found any occurrences of this word in the Greek New Testament.
I believe that
τῇ ἁγιωτάτῃ ... πίστει is the predicative position which would lead to an intensification of the adjective. "Most holy faith" is an interpretation of the emphasis, but not a strict superlative (Jude 20). It's more along the lines of bolding something that you read with emphasis in your head, "... building yourselves up in your holy faith ..."
Hebrews 9:3 is
Ἅγια Ἁγίων which is just "Holy of Holy [ones/things/ideas/places]" which is usually reduced to "Holy of Holies." This is conceptually parallel to the English idea of "The Good, The Bad, and The Ugly." They're substantive adjectives - adjectives that take on the function of a noun. Given the OT imagery that runs through Hebrews, this would have been a natural connection to make to the inner room of the tabernacle.