Holy, holy, holy: To say the word holy twice in Hebrew is to describe someone as “most holy.” To say the word holy three times intensifies the idea to the highest level. In other words, the holiness of God is indescribable in human language. To be holy means to be different, distant, or transcendent. Thus the song of the seraphim is a constant refrain that the transcendence of God is indescribable. Although the Lord is totally different from us—He is perfect—in His mercy He still reaches down to take care of us. The whole earth is full of His glory: The Hebrew word order is, “The fullness of all the earth is His glory.” We know that the glory of God transcends the universe (Ps. 113:4–6); yet in order to balance the expression of the transcendence of God in the first half of this verse, the words of the second half emphasize God’s closeness to His creation—His involvement with the earth and its people.
The NKJV Study Bible. (2007). (Is 6:3). Nashville, TN: Thomas Nelson.