The Trinity is one of the most difficult realities to comprehend but it might be a helpful start to recognize the distinction between a “being” and a “person.”
God is one Being (the one and only divine Being). This divine Being exists as three Persons (Father, Son, and Holy Spirit). This is unusual to us because each human being is only one person and it might seem that every being has to be one person: one being = one person.
But that’s simply not always the case. For example, a dog is a being but is not a person. A tree is a being but is not a person. In these cases, one being = zero persons. On the other hand, an angel is a being and is a person. In their case, angelic beings are similar to human beings: one being = one person.
Once you recognize that not every being is always exactly one person, it might be easier to grasp God’s unique reality, the Trinity: one Being = three Persons.
I hope this is a helpful start. Here is some further reading from Tim Staples.