There have been a couple of recent treatments of the Trisagion -- at least, these are ones I'm aware of.
In 2008 H.G.M. Williamson's "Wellhausen Lecture" was published as Holy, Holy, Holy: The Story of a Liturgical Formula (Walter de Gruyter, 2008).1 He looks at the history of the verse in terms spelled out by the title, but probing back into biblical times (not the later Jewish and Christian "liturgical uses"). His focus is on the second part of the verse, though ("the whole earth is full of his glory") rather than the "Holy holy holy" part in Isa 6:3a.
Partly in response to this, one of the articles in his Festschrift was devoted to the Hebrew Bible's texts that "repeat something three times" (OP).2 In fact, it provides discussion of OP's precise question(s). Some brief responses, then, summarizing from this article, in reverse order:
[OP]: Are there other examples in Isaiah (or elsewhere) of such thrice repetition?
There appear to be just four occurrence of this phenomenon in the whole Hebrew Bible:
- Isaiah 6:3 is the "parade" example supplied by OP;
- Jeremiah 7:4, a slightly "elongated" example, with the three-fold repetition of "the temple of the LORD, the temple of the LORD, the temple of the LORD";
- Jeremiah 22:29: "O land, land, land!"; and
- Ezkiel 21:27: "A ruin, a ruin, a ruin..." (n.b. - this is 21:32 in the Hebrew text).
That appears to be the sum total of them. These were known as a small "corpus" by the medieval rabbinic commentators, as Rashi and Kimchi discussed them in relation to each other.
[OP]: Is there a special meaning to the number three?
There may well be, but this seems not to have a bearing on these triple-repetition texts.
[OP]: I’m wondering about the meaning of repeating three times קָדֹ֛ושׁ (qādôš, "holy"). Is this just an extension of the Hebrew habit of repeating things (usually twice) for "emphasis"?
The argument of the article is that the triple repetition is simply a rhetorical device, providing an unusually insistent focal point on the "thing tripled". The negative side of the argument is that it is, thus, not a "grammatical" or "syntactic" construction per se, although this assertion is sometimes made, especially in light of the handling in Gesenius-Kautzsch-Cowley, § 133k.
For Isaiah 6:3 in particular, the frequent claim that it represents a "superlative of holiness" (p. 215) is also excluded, arguing that:
This ... is the point of the tripling: the three-fold declaration emphatically forces this “otherness” of the LORD into the foreground.
There is, of course, much more said about the linguistic issue, and also about how the analysis of the four texts mutually reinforces this kind of interpretative approach. But it's best to consult the article itself for this.
- Unfortunately, the Google Books "preview" is more like a "tantalizing glimpse", as it only includes a few pages from the Foreword.
- D.J. Reimer, "On Triplets in a Trio of Prophets", in Let us Go up to Zion: Essays in Honour of H. G. M. Williamson on the Occasion of his Sixty-Fifth Birthday, ed. by I. Provan & M. Boda (Brill, 2012), pp. 203-217.