The God of Christianity is said to be a Trinity: three persons, but one Godhead.
The doctrine of the Trinity is not something to be "understood," but rather it is a way of taking a true and incomprehensible statement about God and making it somewhat recognizable to our logical minds. The technical and logical elements of this doctrine are a mystery, and nothing will be gained by trying to understand it through purely logical reasoning. That's because the human mind is ruled by logic, whereas God, creator of all things, is ruled by nothing. God created logic just as he created the universe, and He is therefore above logic, because He existed before logic.
God did, after all, create humans. We cannot understand our own minds because if they were simple enough to be undertood, we would be too simple-minded to understand them. Why, then, do we expect to understand the mind of God?
