Dear Netti-Netti, you have asked: « Why mention all three? Why not simply say "baptise in the name of the One True God"? » and you have deduced with logic: « Based on the fact that Jesus mentions them separately — as though they are to be considered separate entities — I believe we can reasonably conclude that Jesus did not equate them! » when « Jesus said, "Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit" (Matthew 28:19). » [added emphasis and link]
Well, recalling again your previous words, which denote a genuine quest for Truth, and recalling also that no mention to a specific timeframe was made in the Promise; is it possible that each individual will be guided to the KNOWLEDGE that Christ-Jesus spoke of, « I have yet many things to say unto you, but ye cannot bear them now. » (John 16:12), by the Teacher "in spirit and in truth" to come when the individual is found (cf. Luke 15:32) to be ready?: « Howbeit when he, the Spirit of truth, is come, he will guide you into all truth ... [he] shall shew it unto you. » (John 16:13-15)
: Dante:
: Midway upon the journey of our life
: I found myself within a forest dark,
: For the straight-forward pathway had been lost.
: Virgil:
: Therefore I think and judge it for thy best
: Thou follow me, and I will be thy guide,
: And lead thee hence through the eternal place,
--in Divina Commedia by Dante Alighieri, ca. 1304-1321
Furthermore, can it be that, as a new Age (γενεά ?) is now at hand, as the Sun enters Aquarius by precession, as the world and man evolve, the many, instead of the few, are already being individually taught such things? (cf. John 3:9-10)
: The man who realizes his ignorance has taken the first step toward knowledge.
: Christ said, "The Truth shall make you free," but Truth is not found once and forever. Truth is eternal, and the quest for Truth must also be eternal.
-- in The Rosicrucian Cosmo-Conception by Max Heindel, 1909
Can you, dear friend (cf. John 15:13-16), understand now? It is not a question: it is a Call.
« Be not deceived, God is not mocked,
For whatsoever a man soweth, that shall he also reap. »
P.S.: As Thomas has meanwhile transcribed (Disciplina Arcani): "Even The Lord's Prayer was preserved from the knowledge of those who were not fully instructed." [added emphasis and link]
BUT: "At this particular time in the world's work, mankind had reached the stage where it could be given a more exalted phase of the Christian religion and when the mysteries (which the Christ spoke of in Matthew 13:11, and Luke 8:10) should be given to the many instead of the few." (the source)