The third berakha of the Amida is also the shortest one.
In this berakha we say that God is kadosh, and so is His name, and so are the people who praise Him every day: Am Israel, (who imitate the malakhim -angels- mentioned in the Kedusha).
What is 'kadosh'?
First, you need to know that it is impossible to translate the concept of kadosh with one single word. 'Kadosh' means: special; unique; different; consecrated and sometimes: 'out of reach'.
One example: in Hebrew kiddushin means 'marriage'. When a man marries a woman, she is mekuddeshet'consecrated' to him, becoming for him 'unique', 'special' and 'different' from the rest of the women. Now, she is 'out of reach' for all other men.
In the context of our berakha, when referring to God, kadosh hints to our human inability to 'grasp' God 'Almighty-ness'.
ATA KADOSH, means: 'You are beyond our reach', intellectually. God remains 'concealed' to us. No matter how much we praise Him, for those of His actions that WE see, we acknowledge that the full scope of His actions, so to speak, is completely beyond our reach. VESHIMKHA KADOSH, and 'Your name is kadosh'. We state that we are not even capable of grasping His name -the name of four letters name- which conveys the idea of 'infinite' and 'eternity'.
UKDOSHIM BEKHOL YOM: still, we, the kedoshim the people 'consecrated' to You, praise You every day, as much as we are able to praise You. Nevertheless, we remain completely aware that no matter how much we praise You, it will never be enough to 'fully' praise You. (Notice that an identical message constitutes the very core of the kaddish: LEELA MIN KOL BIRKHATA, etc.)
Why do we emphasize this message here, at the end of these blessings (shebach)?
In the two previous berakhot we have praised God's intervention in history (Abot), from our ancestors until the future go'el(mashiach) and we have described and praised God's intervention in nature (geburot): His powers, His care, His miracles.
By saying ata kadosh we convey a sort of a philosophical disclaimer and we virtually 'extend' our praise to 'cover' what we haven't said: What we said of You so far, God Almighty, it is just the little we grasp from our human perspective.
BARUKH ATA HASHEM HA-EL HAKADOSH: Blessed are You, God, Whose real power, blessings and praise remains 'far beyond our reach'.
Rabbi Yosef Bitton. YMJC 130 Steamboat Rd. Great Neck NY 11024