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

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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’m wondering about the meaning of repeating three times קָדֹ֛ושׁ (qādōš, “holy”). Is this just an extension of the Hebrew habit of repeating things (usually twice) for “emphasis”¹? Or is there a special meaning to the number three?² Are there other examples in Isaiah (or elsewhere) of such thrice repetition?

^{1. In quotes because I’m never quite sure what that means. Comments welcome.}

^{2. I did learn the word Trisagion in the process of writing this question, but I’m left without much insight into the point.}