The Doctrine of the Trinity can be expressed in 3 statements. This doctrine is easy to express but impossible to fully comprehend.
1. There exists only 1 being who we call "God."
2. God exists in three beings: the Father, the Son (Jesus Christ) and the Holy Spirit (sometimes called Holy Ghost)
3. All three beings are always existent and are fully divine
There is no explicit expression of this Trinitarian formula, but the three persons are often mentioned in coordination, for example: Rom 1:1-4
"Paul, a servant of Christ Jesus, called to be an apostle, set apart for the gospel of God, which he promised beforehand through his prophets in the holy Scriptures, concerning his Son, who was descended from David according to the flesh and was declared to be the Son of God in power according to the Spirit of holiness by his resurrection from the dead, Jesus Christ our Lord,"
See also: Titus 3:4-6, 1 Pet 1:2, 2 Corinthians 13:14, Ephesians 4:4-6, 1 Corinthians 12:4-6, Ephesians 5:18-20, Jude 1:20-21
When the passages about are coupled with passages expressing God's unity we arrive at the Trinity doctrine. For example: Isaiah 45:5
I am the LORD, and there is no other, besides me there is no God;
See also: Deuteronomy 6:4; John 10:30, 17:3; 1 Corinthians 8:6; 1 Timothy 2:5
I do not hold the KJV translation of 1 John 5:7-8 to be original to the scripture nor inspired by the Holy Spirit.