And one cried unto another, and said, Holy, holy, holy, is the Lord of hosts: the whole earth is full of his glory. (Isa 6:3)
While not a common argument in favour of Trinitarianism, it is not rare to hear some Trinitarians cite as evidence of the Trinity the three-fold repetition of קָדוֹשׁ ("holy") in this verse as being predicated upon three divine persons (notwithstanding "his glory" [Heb. כְּבוֹדוֹ] being third person singular, not plural, indicating a single person is in view within the Hebrew text [same holds true for the LXX which renders the Hebrew as τῆς δόξης αὐτοῦ, literally "the glory of him"]).
Alva Huffer, a systematic theologian of the Church of God, Abrahamic Faith (General Conference), wrote the following which soundly refutes this "argument" for Trinitarianism:
Isaiah 6:3. The seraphim adore God by crying to one another, “Holy, holy, holy, is the Lord of hosts: the whole earth is full of his glory.” The fact that God’s attribute of holiness is repeated three times in the seraphim’s adoration does not indicate the reference is made to three persons of a trinity sitting upon a throne. The word “holy” is repeated three times for emphasis. It means that God is most holy.
Repetition for emphasis is a common practice among Scripture writers. Note the following examples. “O earth, earth, earth, hear the words of the Lord” (Jer. 22:29). Did Jeremiah teach a trinity of earth? Certainly not! “I will overturn, overturn, overturn, it: and it shall be no more until he come whose right it is; and I will give it him” (Ezek. 21:27). God declared that the kingdom of Israel would be suspended and the throne of David overturned. This would remain in effect until the Messiah came to reign as king. In this verse, the word “overturn” is repeated three times for emphasis. (Alva G. Huffer, Systematic Theology [Oregon, Illin.: The Restitution Herald, 1960], 75)