Holy, holy, holy. Figure of speech Epizeuxis for intense and solemn emphasis. Compare the threefold blessing of Numbers 6:24-26 and Revelation 4:8,. threefold unity.
Related Commentaries of Isaiah 6:3
Isaiah 6:3
3 And one cried unto another, and said, Holy, holy, holy, is the LORD of hosts: the whole earth is full of his glory.