God is Three in One. He is one God three parts: God the Father, God the Son and God the Holy Spirit. The three Persons of God are so close they are not three Gods or three separate parts but one God.
Over 2000 years ago God the Son, Jesus, came to live with people. While Jesus lived on earth he healed the sick, fed the hungry and comforted the poor.
Because the Father, the Son and the Holy Spirit are One, when Jesus lived on earth people began to understand more and more what God is like.