What is the significance of calling to each other?
Rashi: They receive permission from each other, lest one precede the others, and be obligated to be burned. Rather, they begin together. This is like we say in the Kedushah in Birkas Yotzer Ohr 'Kulam Onim k'Echad.'
Radak: They make each other zealous.
Targum Yonasan: They receive from each other. Malbim - this is like researchers and those with tradition say, that angels influence each other. They prepare each other to understand the greatness of the Creator and His Kedushah.
Malbim: They inform that Hashem's Kedushah is greater than theirs.
Why do they say "Kadosh" three times?
Targum Yonasan: He is Kadosh in Shamayim elevated above; He is Kadosh on the ground and does His [acts of] Gevurah; He is Kadosh forever 1 . Radak - also Onkelos translates "Hashem Yimloch l'Olam va'Ed" (Shemos 15:18) 'His kingship exists l'Olam ul'Almei Almaya.'
Chulin 91b #1: This shows that Yisrael are dearer to Hashem than the angels. Yisrael say Hashem's Name after only two words - "Shma Yisrael, Hashem... " Angels say His Name only after three words - "Kadosh, Kadosh, Kadosh, Hashem Tzevakos."
Chulin 91b #2: One group of angels says "Kadosh", the next responds "Kadosh, Kadosh", and the third responds "Kadosh, Kadosh, Kadosh, Hashem Tzevakos."
Radak #1: One calls another 'Kadosh Kadosh', like one calls to his friend 'Ploni, Ploni.' They say, let us sanctify Hashem together, and say 'Kadosh Hashem Tzevakos', for He is Master of legions above and below.
Radak #2: All three apply to Hashem. It is repeated three times corresponding to three worlds - the supreme world is angels and Neshamos. The middle world is the spheres and stars. The lower world is this world. The most honored in this world is man. The verse says that He is Kadosh and elevated above three worlds. In two worlds they sanctify and elevate Him with their praises, and also man does so in the lower world 2 .
Radak citing the Kuzari: He is sanctified and elevated from any characteristic of created beings. If a creation is called Kadosh, it is a mere metaphor. Hashem is Kadosh Kadosh Kadosh - without limit. He is elevated from any effect of the Tum'ah of the nation amidst which He made His honor dwell.
Malbim: This is like 'Melech Malchei ha'Melachim.' Hashem is more Kadosh than all Kedoshim; there is no Kedushah like His.
Malbim: I.e. He is Kadosh in Shamayim, for is separated from (has no) form. He is Kadosh in the land, for is separated from physicality. He is Kadosh forever, for He is above absence. The end of the verse explains this. "Hashem" shows eternal Kedushah - He was, is and will be. This corresponds to His own Kedushah. It was the same before He created anything; it never changes. Tzevakos shows Kedushah above in Shamayim - this name is primarily for the legions above, like Ibn Ezra says. This corresponds to miraculous conduct. "Melo Kol ha'Aretz Kevodo" is Kadosh on the land. This corresponds to natural conduct, which is primarily in the land.
Why did Radak not simply say that in all three worlds they sanctify and elevate Him? Perhaps man's sanctification is different, for it is via choice. Or, man knows how to sanctify Him only because he heard how the higher worlds do so; the higher worlds know by themselves. Or, in two worlds, all sanctify Him; in the lower world, only man does. (PF)
Why does it say that His honor fills the entire world?
Radak: He created everything, all those with intellect will honor Him for everything.
Malbim: Even though He has the ultimate Kedushah and elevation, His honor and praise fill also the entire land, which is the lowest extreme of His creation; His Hashgachah fills everything. Nothing is empty of it. Also refer to 6:3:2:1*.
