Ellen White states that the HOLY SPIRIT is both the COMFORTER and the SPIRIT OF TRUTH, while also telling us that the Comforter is the SPIRIT OF CHRIST. And so the “Comforter” and the “Spirit of truth” are both Christ by His Holy Spirit.”
Don't forget that, “The HOLY SPIRIT is the SPIRIT OF GOD; it is also the SPIRIT OF CHRIST. It is that divine, mysterious emanation through which they carry forward their great and infinite work.” — (Uriah Smith, GC Bulletin, March 18, 1891, pp. 146, 147)
“Christ tells us that the HOLY SPIRIT IS THE COMFORTER, and the Comforter is the Holy Ghost, “the Spirit of truth, which the Father shall send in My name.” “I will pray the Father, and He shall give you another Comforter, that He may abide with you for ever; EVEN THE SPIRIT OF TRUTH; whom the world cannot receive, because it seeth Him not, neither knoweth Him: but ye know Him, for He dwelleth with you, and shall be in you” This refers to the omnipresence of the SPIRIT OF CHRIST, called the COMFORTER.” — (E.G. White, 14MR 179.2)
“The HOLY SPIRIT is called both the COMFORTER AND THE SPIRIT OF TRUTH, because there is comfort and hope in the truth.” — (E.G. White, GW92, 310.2, 1892)
“JESUS comes to you as the SPIRIT of TRUTH; study the mind of the Spirit, consult your Lord, follow His way.” — (E.G. White, 2MR 337.1)
“The SPIRIT of TRUTH is the only effectual teacher of divine truth; those who are taught of Him have entered the school of CHRIST. How must God esteem the race, that He gave HIS SON to die for them, and appoints HIS SPIRIT to be man's teacher and continual guide.” — (E.G. White, ST, Oct 24, 1906)
“CHRIST was about to depart to His home in the heavenly courts. But He assured His disciples that He would send them another Comforter, who would abide with them forever. To the guidance of this Comforter all who believe in CHRIST may implicitly trust. HE IS THE SPIRIT OF TRUTH, but this truth the world can neither discern nor receive.” — (E.G. White, 12MR 260.1)
“The HOLY SPIRIT is constantly at work, teaching, reminding, testifying, coming to the soul as a divine COMFORTER, convincing of sin as an appointed judge and guide. CHRIST WAS THE SPIRIT OF TRUTH.” — (E.G. White, SW, Oct 25, 1898)
“How shall I bear impressively the commission CHRIST has given to His people—the privilege of being workers with the SPIRIT OF ALL TRUTH manifest in the flesh—the DIVINE SON OF GOD, clothed with humanity, a channel devised and prepared to be continually receiving and imparting the heavenly current?” — (E.G. White, Ms125, July 25, 1906)
Whose Spirit was sent to guide us into all truth?
“We cannot be with CHRIST in person, as were His first disciples, but HE has sent HIS HOLY SPIRIT to guide us into all truth, and through this power we too can bear witness for the Saviour. [John 16:13 quoted]” — (E.G. White, Ms30, June 18, 1900)
The Spirit of truth is the Spirit of Christ. Thus the Holy Spirit is NOT a third being but the Spirit of God coming to us through His Son.