Where does the Bible say the Father, Son & the Holy Ghost is one person?
John 1:1 says that the Word (Jesus) was with God and was God. In John 10:30 Jesus said that He and the Father are one. In John 14:9 He said that anyone who has seen Him has seen the Father. In Colossians 1:15 Paul wrote that Jesus is the (visible) image of the invisible God. In Hebrews 1:3 Jesus is called the exact representation of God’s glory, and in Hebrews 1:8 God Himself called Jesus God.
God’s Spirit is presumed to be one and the same with God just as your spirit is presumed to be one and the same with you. So if God and Jesus are one and the same, and God and His Spirit are one and the Same, then the three are one.