In our Catholic grade schools we learned that there are three Persons in one God. Further, we learn
that this Trinity is a mystery we cannot fully understand.
However, although we cannot fully understand the Trinity, our life's main concern must be one of our striving to understand the Trinity as far as our feeble minds can go. It is God's Spirit who for us opens a further and further understanding of his Being. And, as Jesus says in today's reading, "God does not ration his gift of the Spirit." In other words, God does not limit the amount about him that we can learn.
That said, let me say what I can about the Trinity.
First, and most important of all, we believe only in one God. Our distinction of Father, Son, and Holy Spirit arise out of our understanding of the three great things he does for us. So, God, as the creator of the universe is the Father; God becoming man to save us is the Son; God communicating with our souls and enriching them, is the Spirit.
Where Jesus In today's Gospel says, "God does not ration his gift of the Spirit" he is telling us we can plunge ever and deeper into our understanding of God and of our pasting of his nature.