The One who is addressed is clear from the following verses.
It is One who 'sits' on the throne.
But we know that God is Spirit. Spirit, unembodied, does not 'sit'. John 4:24.
True, He 'dwelleth the cherubim' but 'between' is an insertion not in the Hebrew. And the ark is a coffer (see the last verse of Genesis) not a sedan chair. 'Dwelleth the cherubim' is a spiritual concept regarding the union of Deity with humanity. Isaiah 37:16, 2 Kings 19:15, Psalm 99:1.
But in the midst of the throne is the One who is 'the Lion of the tribe of Judah'. It is the Lamb who is able to open the book that is in the hand of 'him who sat on the throne'.
And yet the Lamb is 'in the midst of the throne'.
And the Lamb - who is in the midst of the throne - has seven horns and seven eyes which are the seven Spirits of God.
The whole vision, of one who is 'holy, holy, holy Lord' is a matter of embodiment and enthronement.
Enthroned, is One who has a book, and in the midst is one who takes the book, and part of he, himself, is a seven-fold expression of Spirit.
This is the One who is 'holy, holy, holy'.
The grammar, alone, does not explain the meaning. 'Holy' is an adjective. And three adjectives are supplied.
And three Persons are described, in vision.
And these three are 'One'. The 'One' who sits, is seated, in humanity, God over all, blessed for ever, Romans 9:5.
'Lord' is a form of address. 'God' is a concept. 'Almighty' is a description.
The Lord who is 'holy, holy, holy' is one Lord.
There is 'glory in the Church, by Jesus Christ' to Him who is able to do above all that we ask or think, Ephesians 3:21. And those who are gathered are urged to unity : the unity of One Spirit, Ephesians 4:3.
Now, 'the Lord is that Spirit' 2 Corinthians 3:17. 'One Lord, one faith, one baptism, one God and Father of all,' Ephesians 4:4-6.
Yet, here, in the midst of a throne upon which 'One', who is Spirit, sits -there is a Lamb. And part of Him, is an embodiment of the seven-fold Spirit.
And the 'Lord' is said to be 'holy, holy, holy'.
This is 'the God' (the Deity) who is 'Almighty'.