God is uniquely one, yet triune -- the Father, the Son, and the Spirit.

1 Timothy 2:5a For there is one God...
Matthew 28:19 ...the name of the Father and of the Son and of the Holy Spirit

God is a mystery! We can never fully understand it, but we can simply receive what the word says about God, and enjoy it! There are several things the word makes clear:

I. God being only one
How many Gods do you have? One!
(1 Tim. 2:5, Isa. 45:5, 1 Cor 8:4, and many more)

II. God being triune--Father, Son, and Spirit
Although God is one, He has an aspect of three. This is why we call Him triune (tri meaning 3 and une meaning 1 in Latin). The Father is God, the Son is God, and the Spirit is God, yet they are all one God. Haha, amen!

It's tempting to try to explain this logically. But don't do this! You will fall into a trap. Instead, realize these points:

A. The Father, Son, and Spirit are all eternal
The Son did not come into existence at some point in time. He is without beginning and without end (eternal).  The Father and the Spirit are also both eternal. 
(Isa. 9:6, Heb. 7:3, Heb. 9:14)

B. All three coexisting and not being in modes (stages)
The Father didn't morph into the Son, who later morphed into the Spirit. They all exist all the time, at the same time! 
(Matt. 3:16-17, John 14:16-17, Eph. 3:14-17)

C. All three coinhering and not being three separate Gods
Coinhering means that they live within one another.  John 14 clearly says that the Father is in the Son, and the Son is in the Father. Also, the Father and the Son come to us as the Spirit.
(John 14:10-11, 17-18, 10:30)

So accept the mystery! When you receive the Spirit, you receive all of God! When you call Lord Jesus, you also get all of God! God is triune for His dispensing! Hallelujah!