Q: How many persons are there in the Godhead?
A: There are three persons in the Godhead; the Father, the Son, and the Holy Ghost; and these three are one God, the same in substance, equal in power and glory.1

2 Corinthians 13:14. The grace of the Lord Jesus Christ, and the love of God, and the communion of the Holy Ghost, be with you all. Amen.
Matthew 28:19. Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.
Matthew 3:16-17. And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him: and lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.

Westminster Project - For more info take a look at John Flavel's expostion and proofs for the WSC.