Was Matthew 28:19 . . .

I haven't ever heard that so I cannot say for absolute sure but what I can say is that my copy of the Greek New Testament (which is the very respected version that Bruce M. Metzger worked on) does include tou patros dai tou huiou kai tou agiou pneumatos which is the phrase that you are asking about.

In case you don't know, unlike in publications of the Hebrew Bible in the original language (which works off of one complete manuscript), the Greek New Testament is compiled from a variety of the best manuscripts around. Then at the bottom of the page it lists which manuscript the text was taken from if there are any significant textual variances (that is where the text they have chosen may differ in other manuscripts).

In regards to this passage it does not list that there are any manuscripts with any textual variances. As such, I would be willing to bet that the idea that Constantine added this is a folklore, probably (and I am not blaming you for this) in an attempt to discredit either trinitarian belief or the Catholic Church which is often associated with that phrase though virtually every group of Trinitarian Christians use it in baptisms at the very least.

Those who deny the authenticity of Mattew 28:19 are simply trying to bolster their anti-trinitarian positions. This is silly, of course, since we can easily show evidence for the Trinity without using 28:19. That said, every mss of Matthew without exception contains the trinitarian reading. Though (to my knowledge) all of these date later than 300AD, it is nothing short of baseless speculation to argue that "the Father, the Son, and the Holy Spirit" is inconsistent with the autograph. I've been hit with this gibberish before. My most frequent response to those who insist 28:19 did not appear in mss before Constantine is, "PROVE IT!"

Ploughman and others have tried their level best to argue around trinitarian baptism, but their arguments fail every time. They disregard or skew the broader testimony of Scripture concerning the divinity of Christ and the Spirit. ALL textual evidence -- internal and external -- points toward the authenticity of 28:19.

Hope this helps.