It is the threefold repetition of the word holy, ascribed to God in Isaiah 6.
Tree sigh on
Called a 'trihagion,' it is found in the Old and New Testaments here:Isaiah 6:3New International Version (NIV)3 And they were calling to one another:"Holy, holy, holy is the Lord Almighty;the whole earth is full of his glory."Revelation 4:8New International Version (NIV)8 Each of the four living creatures had six wings and was covered with eyes all around, even under its wings. Day and night they never stop saying:"'Holy, holy, holyis the Lord God Almighty,'who was, and is, and is to come."