Ok, lets take a second look at what scripture says...
"In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God."
^ so here we have two distinct persons within the one God (the word who was with God, and who is God; and also God, who was with the word, and who is God)
And who is this word?
"And the Word became flesh, and dwelt among us, and we saw His glory, glory as of the only begotten from the Father, full of grace and truth. John *testified about Him and cried out, saying, “This was He of whom I said, ‘He who comes after me has a higher rank than I, for He existed before me.’”
^ so here we see that the word who was with God, and who is God, is the only begotten son of God, who John testified of. clearly, the word who is God and who was with God, the same one is Jesus, the only begotten son of God that John testified about.
So now we know that in the one God, there are two, both the word (who is Jesus), and God.
now, why should we consider that there may be another person of the God head? well, Jesus Himself elevated The Holy Spirit to that of God, when He commanded us to baptize in the name of The Father, The Son, and The Holy Spirit. but other than that, what proof do we have from the bible that the Spirit of God is indeed God, in the same way that the son of God is God? well, we see in Acts chapter 5, verses 3 and 4 that Peter says that lying to The Holy Spirit is lying to God.
"But Peter said, “Ananias, why has Satan filled your heart to lie to the Holy Spirit and to keep back some of the price of the land? While it remained unsold, did it not remain your own? And after it was sold, was it not under your control? Why is it that you have conceived this deed in your heart? You have not lied to men but to God.”
So we have The Father, The Son, and The Holy Spirit. When God made man, He said "Let us make mankind in our image". The Father, The Son, The Holy Spirit.