First of all, it is the Church’s teaching and its deepest experience that there is only one God because there is only one Father.
In the Bible the term “God” with very few exceptions is used primarily as a name for the Father. Thus, the Son is the “Son of God,” and the Spirit is the “Spirit of God.” The Son is born from the Father, and the Spirit proceeds from the Father—both in the same timeless and eternal action of the Father’s own being.
In this view, the Son and the Spirit are both one with God and in no way separated from Him. Thus, the Divine Unity consists of the Father, with His Son and His Spirit distinct from Himself and yet perfectly united together in Him.