The Catechism in Questions & Answers

51. Who is the triune God?

God is a spiritual, perfect, and completely independent being. He is eternal, without beginning and without end. The one God is the Father, the Son, and the Holy Spirit.
When we talk about “the Father, the Son, and the Holy Spirit”, we are not referring to three different gods, but rather three persons who are the one God.

The Trinity of God: see Questions 61. et seq., 198.