Answer to the question:

How many Beings were present at Christ's baptism?

Answer. Two Beings. The Father and Son.

Matthew 3:16,17 And when Jesus was baptized, he went up immediately from the water, and behold, the heavens were opened and he saw the Spirit of God descending like a dove, and alighting on him; and lo, a voice from heaven, saying, "This is my beloved Son, with whom I am well pleased."

Here, we are told, there were the three members of the godhead all in one place. The Son was baptized, the Father spoke, and the Holy Spirit descended on the Son in the form of a dove. Let us see how Ellen White explains this passage.

“Never before had angels listened to such a prayer as Christ offered at His baptism, and they were solicitous to be the bearers of the message from the Father to His Son. But, no! Direct from the Father issues the light of His glory. The heavens were opened and beams of glory rested upon the Son of God and assumed the form of a dove, in appearance like burnished gold. The dovelike form was emblematical of the meekness and gentleness of Christ. . . . From the opening heavens came these words: “This is my beloved Son, in whom I am well pleased.” . . . Notwithstanding the Son of God was clothed with humanity, yet Jehovah, with His own voice, assures Him of His sonship with the Eternal. In this manifestation to His Son, God accepts humanity as exalted through the excellence of His beloved Son”. --That I May Know Him, p. 31

'John came bearing witness of the One whose divinity they were now questioning. Christ himself had gone to the Jordan, not to repent of sin, but to fulfill every specification required of the sinner. The Baptist saw the Saviour walking at a distance, and his face lighted up. "Behold the Lamb of God," he cried, "which taketh away the sin of the world." There Christ stood revealed before the people. The glory of God descended upon him in the form of a dove like burnished gold, and the voice of the infinite One declared, "This is my beloved Son, in whom I am well pleased."' --Review and Herald, February 13, 1900 par. 14

'After Christ was baptized of John in Jordan, he came up out of the water, and bowing upon the banks of the river, he prayed with fervency to his Heavenly Father for strength to endure the conflict with the prince of darkness in which he was about to engage. The heavens were opened to his prayer and the light of God's glory, brighter than the sun at noonday, came from the throne of the Eternal, and, assuming the form of a dove with the appearance of burnished gold, encircled the Son of God, while the clear voice from the excellent glory was heard in terrible majesty, saying, "This is my beloved Son in whom I am well pleased."' --Signs of the Times, August 7, 1879 par. 7

Here we see that there is a Father, Son and Spirit of God. But what is the Spirit of God? The Spirit of God is the glory of God. It is the Father's glory that assumed the form of a dove and encircled the Son of God.