Many conclude that the dove is the third person of a Trinitarian Godhead consisting of three co-equal and co-eternal Persons, but let us read another passage of the same event. 

“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’.” IHP p39. ST Aug 7. 1879. 

Note, it is the beams of glory from the throne that assumed a dove-like. The dove is not a third God-Being called God the Holy Spirit. The light that surrounds the Father is the divine Spirit – the Holy Spirit -- that anointed His Son at the beginning of His ministry. “The prayer of Christ in behalf of lost humanity cleaved its way through every shadow that Satan had cast between man and God, and left a clear channel of communication to the very throne of glory. 

Don't miss these great eye opening sites below: