The trinity doctrine is an essential doctrine for any valid Christian denomination to contain. The doctrine of the trinity is that there is one God in three persons; the Father, the Son/Word (Jesus), and the Holy Spirit. The best explanation of the trinity doctrine in greater detail comes from the Athanasian Creed. The primary verses for the trinity doctrine are John 1:1-14 that establishes that the Word made flesh, Jesus, is also God. This lines up then with Genesis 1:1-3 that shows the trinity doctrine with the Father creating the universe by his Word and his Holy Spirit and thus showing that Jesus is eternal and not a created being as was put forth by the heresy of Arianism or elsewise entirely different religions that just wholly deny the divinity of Jesus. Some newer sects will maintain that Jesus is eternal and is also God but will then confuse the trinity doctrine with modalism, which confuses the distinct three persons of the trinity together into one God with three modes. To make the trinity doctrine sure and also counter the heresy of modalism then the baptism of Jesus account, especially in Luke 3:21-22, shows clearly the truth of the trinity doctrine in that all three persons of the trinity are present and distinct and acting simultaneously rather than God being in three modes. Apart from these there are of course other scriptures that name the three persons of the trinity in passing (Paul's letters) or indirectly support the trinity doctrine (prophets of the Old Testament) and are supporting proofs, but these are the three best scriptures that form the scriptural backbone of the trinity doctrine.
John 1:1-14
1 In the beginning was the Word, and the Word was with God, and the Word was God.
2 The same was in the beginning with God.
3 All things were made by him; and without him was not any thing made that was made.
4 In him was life; and the life was the light of men.
5 And the light shineth in darkness; and the darkness comprehended it not.
6 There was a man sent from God, whose name was John.
7 The same came for a witness, to bear witness of the Light, that all men through him might believe.
8 He was not that Light, but was sent to bear witness of that Light.
9 That was the true Light, which lighteth every man that cometh into the world.
10 He was in the world, and the world was made by him, and the world knew him not.
11 He came unto his own, and his own received him not.
12 But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name:
13 Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God.
14 And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
Genesis 1:1-3
1 In the beginning God created the heaven and the earth.
2 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
3 And God said, Let there be light: and there was light.
Luke 3:21-22
21 Now when all the people were baptized, it came to pass, that Jesus also being baptized, and praying, the heaven was opened,
22 And the Holy Ghost descended in a bodily shape like a dove upon him, and a voice came from heaven, which said, Thou art my beloved Son; in thee I am well pleased.