Notes from Matthew 3:16, 17
The doctrine of the Trinity, which appeared later in church history, teaches that God is three persons and yet one in essence. In this passage, all three persons of the Trinity are present and active. God the Father speaks; God the Son is baptized; God the Holy Spirit descends on Jesus. God is one, yet in three persons at the same time. This is one of God's incomprehensible mysteries. Other Bible references that speak of the Father, Son, and Holy Spirit are Matthew 28:19; John 15:26; 1 Corinthians 12:4-13; 2 Corinthians 13:14; Ephesians 2:18; 1 Thessalonians 1:2-5; and 1 Peter 1:2.
Notes from Mark 1:10, 11
The Spirit descended like a dove on Jesus, and the voice from heaven proclaimed the Father's approval of Jesus as his divine Son. Here we see all three members of the Trinity together--God the Father, God the Son, and God the Holy Spirit. (See also Matthew 28:19; Luke 1:35; John 15:26; 1 Corinthians 12:4-13; 2 Corinthians 13:14; Ephesians 2:18; 1 Thessalonians 1:2-5; 1 Peter 1:2.)
Notes from Acts 1:4, 5
The Trinity is a description of the unique relationship of God the Father, the Son, and the Holy Spirit. If Jesus had stayed on earth, his physical presence would have limited the spread of the Good News because physically he could be in only one place at a time. After Christ was taken up into heaven, he would be spiritually present everywhere through the Holy Spirit. The Holy Spirit was sent so that God would be with and within his followers after Christ returned to heaven. The Spirit would comfort them, guide them to know his truth, remind them of Jesus' words, give them the right words to say, and fill them with power (see John 14-16).