Why is it that in the third canticle, we say things such as bless the Lord O sun, moon, clouds, snow, heat etc. How can non living things bless the Lord?
The third canticle is the praise sung by the three Young Men in the Fiery Furnace. It is found in "completion of the book of Daniel" in the so-called Deuteron-canonical books omitted by the Protestants. This canticle is a praise of the whole creation. David the Prophet also invited all creation to bless the Lord, "Bless the LORD, all His works, In all places of His dominion" (Psa 103:22). The idea is not that non living things bless the Lord but that they continually, by day and night, present themselves to our view. Each one has something special to it for which to praise God. When we look at God's creation, we see the glory of God and it is an invitation for us to bless Him and praise Him always. Also their obedience to the Lord in following His orders is their way of blessing Him.