If Christ was God the Father, and God the Son, and God the Holy Spirit; how could a voice from heaven say, "This is My beloved Son," when he was a man on earth?

This is an age of general enlightenment. Knowledge is increasing along all lines, and specially so in regard to the teachings of the Scriptures. God’s Word has been woefully misrepresented both by friends and foes. The expression of the creeds contained in the question is nowhere found in the Bible. Our Lord Jesus never claimed to be “God the Father” but always said he was the son of God. “My Father is greater than I” (John 14:28); “God sent His son into the world (1 John 4:9).” Our Lord Jesus was not his own Father; but as the Scriptures declare, he “was the beginning of the creation of God (Revelation 3:14).” The voice from heaven to the son on earth is just one evidence that the heavenly Father and our Lord Jesus are two distinct personalities.

Additional Resource:
For a more detailed study of God, Jesus and the holy spirit, download the free PDF: “Father, Son and Holy Spirit”