The word "Bible" isn't in the Bible either...The term "trinity" is not in the Bible, but the concept is:
Matthew 3:16 And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:
Matthew 3:17 And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.
Matthew 28:19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
2 Corinthians 13:14 The grace of the Lord Jesus Christ, and the love of God, and the communion of the Holy Ghost, be with you all. Amen.
1 John 5:7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
1 John 5:8 And there are three that bear witness in earth, the spirit, and the water, and the blood: and these three agree in one.
The popular hymn Holy, Holy, Holy proclaims, "Holy, Holy, Holy, Lord God Almighty! . Merciful and Mighty! God in Three Persons, blessed Trinity."
The OT prophet Isaiah recognized the triunity of God:
Isaiah 6:1 In the year that king Uzziah died I saw also the Lord sitting upon a throne, high and lifted up, and his train filled the temple.
Isaiah 6:2 Above it stood the seraphims: each one had six wings; with twain he covered his face, and with twain he covered his feet, and with twain he did fly.
Isaiah 6:3 And one cried unto another, and said, Holy, holy, holy, is the LORD of hosts: the whole earth is full of his glory.
Isaiah 6:4 And the posts of the door moved at the voice of him that cried, and the house was filled with smoke.
Isaiah 6:5 Then said I, Woe is me! for I am undone; because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips: for mine eyes have seen the King, the LORD of hosts.