Comparison: There are no changes from the Matthean version. However, the very fact that this phrase is included provides more evidence for Joseph’s interaction with the King James Version in his Book of Mormon translation. The late Raymond E. Brown, Auburn Distinguished Professor Emeritus of Biblical Studies at Union Theological Seminary in New York City, provided background:
The KJV of Matt. 6:13 has a doxology or ascription concluding the Lord’s Prayer, “For thine is the kingdom, and the power, and the glory, for ever. Amen,” drawn from the (inferior) Greek mss. used in the translation history of that version. The clause was lacking in Jerome’s Vulgate on which Roman Catholic translations were based; and so there developed an ecumenical problem: In the English-speaking world there were two different ways to end the Lord’s Prayer. Today the great majority of text-critics recognize that the ascription was not written by the Matthean evangelist but was an ancient expansion for liturgical use based on I Chron. 29:11.
On this basis, those who see Joseph Smith as translating this 3 Nephi text directly from the plates therefore sometimes claim that he was no translator at all, having “translated” a phrase that was probably not original to Matthew.
Of course, such an approach entirely misses the point. Joseph’s translation method did not always require him to translate known texts anew from the plates. He obviously took the position that it was preferable for his audience that texts resembling the King James Version also replicate its language. The results are not evidence against Joseph’s translation, but rather evidence for Joseph’s particular definition of translation. Since he used King James Version Matthew as the basis for the 3 Nephi sermon, it is no surprise that he retained this phrase.