The Trinity and the work of the trinity in our salvation is laid out in this one verse. {1Pe_1:2} Those that wish to deny the Trinity would have to explain away the clear words found here.
God the Father
sanctification of the Spirit
the blood of Jesus Christ
The Trinity at the best of times is a challenge to explain.
I heard it explained this way: There are three who's; Father, Jesus, & Holy Spirit and one what GOD.
The Bible is clear when it speaks of: God the Father, God the Son, and God the Holy Spirit.
But states that there is only one God
"Hear, O Israel! The LORD is our God, the LORD is one!" Deu_6:4 cf Mar_12:29
If we were to use math, it would not be, 1+1+1=3 this would be polytheism.
It would be 1×1×1=1 this is monotheism. God is a Triune God.
Trinity is clearly shown in the bible. See Act_2:32-33, Act_7:55, 2Co_13:14, 1Pe_1:2.
The Trinity is clearly seen in the baptism of Christ Jesus Luk_3:22
So to answer your question; who is the one true God? God is a Trinity so whichever you speak of; Father, Son or Spirit you are speaking of the one true God.
I hope this helps clear up a hard to grasp concept.