This point is key, and is made by a number of scholars over the years.Apostolic Friends Forum
The traditional Matthew 28:19 is an interpolation.
Brother Avery mentioned this (above):
An Introduction to the New Testament: Containing an Examination of the Most Important Questions Relating to the Authority, Interpretation, and Integrity of the Canonical Books, with Reference to the Latest Inquiries, Volume 1 (1848)
Hence the expression implies the idea of an engagement to believe in Father, Son, and Spirit. But a combination of the threefold view of God implying such reflection, though it might indeed be found in the apostles (2 Cor. xiii. 13), could hardly appear in Christ, and even among the former it could scarcely be exhibited as an object of confession.
Samuel Davidson (1806-1898)
Here, a common argument is made that Jesus couldn't have said what the Bible says He said, because Jesus wasn't a trinitarian, and trinitarianism came around much later. The author quoted above claims the trinitarian view of God might be found in the apostles but not at all in Christ.
But this is really a sign of shoddy thinking on Mr Davidson's part.
Matthew was an apostle, his Gospel is an apostle's presentation of the doctrine of Jesus. No less than Paul's writings are likewise.
Secondly, and more importantly, Jesus speaking of Father, son, and Spirit has nothing to do with a "three fold view of God". Although many Oneness people think of God as a trinity of "aspects" or "modes" in one Person, this type of thinking is actually a reaction to trinitarian error, and does not fully reflect the original apostolic understanding.
By Father, Jesus and His disciples would understand "Jehovah", Adonai, Kurios, the LORD, the God of Abraham, Isaac, and Jacob.
By Son, they would understand the Messiah, Son of David, Son of Adam, Son of Man (Dan 7), and Son of God (Psalm 2:7, Psalm 89:27, 2 Sam 7:14).
By Spirit, they would understand the anointing, empowering, sanctifying, guiding, controlling presence of God (see Numbers 11:25-29, 1 Sam 10:6-7, Ezekiel ch 3, Ezekiel 36:26-27, Joel ch 2, etc).
There was ONE NAME that conjoined all these (God, Messiah, and Spirit), and that name is JESUS CHRIST. See Isaiah 9:6. Jesus is the Son/Messiah (clearly a human being), but also the Father (God manifest in flesh), and indeed the Holy Spirit (primarily in the sense of the Pentecostal empowerment the believers would have during this life, but also in the overall sense of the power and activity of God-interacting-with-Creation, etc).
So Jesus wasn't speaking in trinitarian terms, nor was Matthew, nor was Paul (or John or anyone else). But neither were they speaking or thinking in some kind of post-trinitarian reactionary modalistic framework, either. Once this is realized, the objections to the Lord's words (like the one quoted above) are driven away like so much chaff.