In the Sanctus we repeat the words spoken in Isaiah 6 by the seraphim, “Holy, holy, holy is the Lord of hosts; the whole earth is full of his glory!”
Isaiah's response is one of total fear, for he knows that he is a man of unclean lips and he has seen the Lord of hosts. Isaiah truly and honestly believes he is about to die as he hears these words of the angels.
And here we are, centuries later, repeating the words Isaiah heard and they no longer cause fear and trembling. They are words of joy.
Of course we add on to them a bit. In many versions of the Sanctus we add words spoken on Palm Sunday by the crowds, "Hosanna (save us now) in the highest. Blessed is He who comes in the name of the Lord."
Again, this may seem an odd choice. After all, Jesus' triumphal entry into Jerusalem is an entry of peace (riding on a donkey as opposed to a warhorse), but it is not the peaceful entry we expect. Jesus makes peace by the blood of His cross (Colossians 1:20).
The Sanctus seems to be two random pieces of Scripture taken out of context and forced together in a rather odd spot in the liturgical movement.
It's like in The Lord of the Rings films when they take quotations from some characters and give them to other characters. This should fail miserably, but somehow it ends up working out just fine. People who have not read the books probably have no idea these lines were re-assigned so to speak. The films take lines from Tom Bombadil and give them to Treebeard, lines from Gandalf and give them to Grima Wormtongue, lines from Faramir and give them to Eowyn. If you watch the appendices on the extended editions of the films, the writers will justify and defend these moves, noting the importance of Tolkien's language and wanting to use it somewhere.
I remember going to a Zac Brown Band concert where they played Bohemian Rhapsody by Queen. It should not have made sense, a country band performing this 1970s rock ballad, but it was perhaps their greatest performance of the night.
Perhaps you really enjoy pizza with odd flavor combinations that should not make sense. I remember having one with asparagus and sausage that was pretty good.
In each case what doesn't seem to make sense at first glance ends up working quite well.
That's sort of how I feel about the Sanctus. The creators of the liturgy in centuries past wanted to use Isaiah 6 and they wanted to use the Palm Sunday "Hosanna" language. They decided to put them together at this moment of praise in the liturgy and it shouldn't work, but it does.
It is fitting for us to call holy, holy, holy in the moments before approaching the altar of the Lord. It is fitting for us to call for God to save us now (Hosanna) before we receive Jesus body and blood for our forgiveness, life, and salvation.