What the author is doing by using this phrase is referring to a major aspect of Christian theology. He is showing that he believes in the Christian idea of the Trinity.
In Christian theology, there is only one God. But, in one of what is called the "mysteries of faith" that one God is actually made up of three "persons." They are somehow separate but yet they are all one God. In this theology, there is God the Father, God the Son (Jesus Chris) and God the Holy Spirit -- as one Christian hymn goes, "God in three persons, blessed Trinity."