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Why would Jesus tell the apostles to baptize in the name of “Father, Son and Holy Spirit” then (Matt. 28:19)?

Emphasizing what you have said, The risen Jesus' command to the eleven disciples was to baptize disciples made by them:

". . . το ονομα του πατρος και του υιου και του αγιου πνευματος." (Textus Receptus).

το ονομα = The Name (in Hebrew this 'HaShem' = 'The Name' is substituted for יהוה , translated as 'Jehovah' in the KJV); and singular, not plural in the grammar of Hebrew or Koine Greek

του πατρος = of The Father (as a title of the paternal relationship to His created things, genitive possessive regarding the Name of God)

και = and (copulative conjunction)

του υιου = of The Son (another title, a second designation, that of the relationship of the Divine-become-human to The Father, possessive regarding the Name of God)

και = and (iterated copulative conjunction)

του αγιου πνευματος = of The Holy Spirit (yet a third title designating the relationship of the angelic phase of God, genitive-possessive regarding the Name of God)

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In Hebrew, HaShem, The Name (singular) of The God is 'יהוה'

In Koine, To Oνομα, The Name (singular) of The God is 'θεος'

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This command means to baptize 'in the Name of"; that is, to baptize "with all the delegated authority of" =======

Semantically, this is not different than to say that the compound of three atoms of the elements, identified by letters of the alphabet as H₂O, can be described as 'ice' (solid phase ), as 'water' (liquid phase), and/or as 'steam' (gaseous phase); but they are all of the same substance, and can coexist in equilibrium each with the other two, yet each phase is physically distinct from the other phases.

In the same way, The Father is a distinct form of יהוה , Jesus of Nazareth The Son is a distinct form of יהוה , and The Spirit is a distinct form of יהוה ; but all are יהוה in substance. They are also named together as אלהים (Elohim), the Uniplural God. Thus, The LORD God -- יהוה אלהים -- is a Trinity, a Being with three facets, three Persons, but of One Continuous Substance. =======

We do not need to get into n-dimensional mathematics to explain this portion of the existence of The Almighty Creator God Who existed before space, time, and mass.