What is the meaning of the repetition of “holy” in Isaiah 6:3?

In Isaiah 6:3, the seraphim call one to another, saying: Holy, holy, holy is the LORD of hosts; the whole earth is full of his glory!” (ESV) קָדֹ֧ושׁ׀ קָדֹ֛ושׁ קָדֹ֖ושׁ יְהוָ֣ה צְבָאֹ֑ות מְלֹ֥א כָל־הָאָ֖רֶץ כְּבֹודֹֽו׃ (BHS) I…