This verse is a companion to the promise that the law will not be destroyed. The promise extents not to simply the overall structure, but to the small pieces, the “jots” and “tittles.” This is a parallel to the fulfillment phrase in the previous verse that is added for emphasis.
Textual: The KJV translation uses “jot” and “tittle.” The Greek text uses iota and “hook.” The iota is the smallest Greek letter, corresponding to the Hebrew yod (transliterated through translations into “jot”). The “hook” was an ornamental touch added to some Hebrew consonants. (Robert Guelich. A Foundation for Understanding the Sermon on the Mount. Word Publishing, Dallas. 1982, p. 144). In both cases, the KJV elects to look past the Greek and illustrate a more Hebrew conception, a trait retained in the 3 Nephi redaction.
The changes in the 3 Nephi text reflect a difference between promise and fulfillment that is appropriate to the context of the Old World and New World Sermons as they stand on either side of Jesus Atonement. In the Old World the fulfillment was yet to come in the perfect sacrifice. In the New World it has already occurred.
Matthew 5:18
18 For verily I say unto you, (Till heaven and earth pass,) one jot [nor] (or) one tittle [hath not passed away] (shall in no wise pass) from the law, [but in me it hath all been] (till) all be fulfilled.
The most interesting change is the removal of the phrase “till heaven and earth pass” from the Matthean Sermon as it was translated into the 3 Nephi account. While interesting, all possible explanations are speculative, and not necessarily pertinent to the message of the text. It is noted that the change was made, but we shall leave it without attempted explanation.