The Trinity

Besides the perpetual controversy between "Oneness" and "Trinitarian" Christians, many people unfamiliar with the Word of God have no clue about "the Godhead." (Col. 2:9; Acts 17:19; Rom. 1:20) Nevertheless, most all fundamental Christian groups hold the doctrine (teaching) of the Trinity; the Father, Son and Holy Ghost, as three separate persons in the "Godhead". No special revelation is required to comprehend the Trinity.

Oneness

God revealed Himself three different ways at three different times
Trinitarians; God so loved the world that He gave His only Begotten Son (Jn. 3:16)
Later that Son would ask the Father to send "another" to comfort the disciples, since Jesus was leaving them alone and they feared the future. See John 14 (16) and see also John 14 (25-28) below.

John 14 (25-28)
25 "These things have I spoken unto you, being yet present with you. 26 But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you. 27 Peace I leave with you, my peace I give unto you: not as the world giveth, give I unto you. Let not your heart be troubled, neither let it be afraid. 28 Ye have heard how I said unto you, I go away, and come again unto you. If ye loved me, ye would rejoice, because I said, I go unto the Father: for my Father is greater than I."

I John 5 (6-9)
6 "This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth. 7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. 8 And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one. 9 If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son."

 

Note
The Spirit is an eyewitness of how Jesus came
There is more than one witness, in fact three, whose testimonies agree
These testimonies are from God, not men who could lie
God the Father also testified to these things

Jesus thought of it this same way, when He told that Pharisees that they knew Him, but did not know the one that sent Him; He also stated that He did not send Himself.

John 7 (28-30)
28 "Then cried Jesus in the temple as he taught, saying, Ye both know me, and ye know whence I am: and I am not come of myself, but he that sent me is true, whom ye know not. 29 But I know him: for I am from him, and he hath sent me."

 

John 15 (23-27)

Note the key words here; 23 "He that hateth me hateth my Father also. 24 If I had not done among them the works which none other man did, they had not had sin: but now have they both seen and hated both me and my Father. 25 But this cometh to pass, that the word might be fulfilled that is written in their law, They hated me without a cause.
26 But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me: 27 And ye also shall bear witness, because ye have been with me from the beginning.