Charles Ellicott Commentary Isaiah 6:3

"And one cried unto another, and said, Holy, holy, holy, is Jehovah of hosts: the whole earth is full of his glory." — Isaiah 6:3 (ASV)

And one cried to another. —So in Psalm 29:9, which, describing a thunderstorm, favors the suggestion that the lightnings were thought of as the symbols of the fiery seraphim, we read, in his temple does every one say, Glory. The threefold repetition, familiar as the Trisagion of the Church’s worship, and reproduced in Revelation 4:8 (where Lord God Almighty appears as the equivalent of Jehovah Sabaoth), may represent either the mode of utterance, first antiphonal, and then in full chorus, or the Hebrew idiom of the emphasis of a threefold iteration, as in Jeremiah 7:4; Jeremiah 22:29.

Viewed from the standpoint of a later revelation, devout thinkers have naturally seen in it an allusive reference to the glory of Jehovah as seen alike in the past, the present, and the future, which seems the leading idea in Revelation 4:8, or even a faint foreshadowing of the Trinity of Persons in the Unity of the Godhead. Historically, we cannot separate it from the name of the Holy One of Israel, which, with the Lord of hosts, was afterwards so prominent in Isaiah’s teaching.