Herman Witsius says that “Isaiah saw the whole Trinity, like a king sitting on a throne” and points not only to the more prominent passage of John 12:41, but also points to Acts 28:25-27, where Paul quotes the same prophecy about the hardness of heart from Isaiah 6 and says, “The Holy Spirit was right in saying to your fathers through Isaiah the prophet.” Witsius clearly reasons that if Isaiah saw the glory of the Son and heard the speech of the Holy Spirit and, “I imagine, none should excluded the Father.”
Isaiah’s famous vision report in chapter six opens thus:
In the year that King Uzziah died I saw the Lord sitting upon a throne, high and lifted up; and the train of his robe filled the temple.
Obviously, the “Lord” is God. But the question is, “who” are we seeing? Classically, it’s been common for Christian commentators to hear a reference to the whole Trinity in the Trisagion, “Holy, holy, holy is the Lord of hosts; heaven and earth are full of your glory” (v. 3).
Cyril of Alexandria writes of the blessings which fill the mouth of the Seraphim:
They say “holy” three times and then conclude with “Lord of hosts.” This demonstrates the holy Trinity exists in one divine essence. All hold and confess that the Father exists, along with the Son and the Spirit. Nothing divides those who are named nor separates them into different natures. Just the opposite is true. We recognize one Godhead in three persons.
Theodoret of Cyrus comments similarly that as the seraphim praise “the title Lord singularly in this song, but repeat ‘holy’ three times (in reference to the Trinity), we know they are referring to the one essence of Deity.” If you see the tripling of ‘holy’ as a reference to the Trinity, then the praises of the seraphim seem to clear that up very quickly.