We express the doctrine of the Trinity basically by saying that there is one God in three persons who are equal in substance and glory. The Westminster Shorter Catechism says:
Question 5. Are there more gods than one?
Answer: There is but one only, the living and true God.
Question 6. How many persons are there in the Godhead?
Answer: There are three persons in the Godhead: the Father, Son, and Holy Spirit; and these three are one God, the same in substance, equal in power and glory.