The idiom "三缄其口" is formed by sequentially combining the character "三" (meaning "three") as an adverbial modifier for the verb "缄" (meaning "to seal"), which then governs the object phrase "其口" (composed of the possessive particle "其" meaning "its" and the noun "口" meaning "mouth"), resulting in a literal structure that denotes "to seal its mouth three times" and functions as a fixed verbal expression.