Textual: These verses are completely new in the 3 Nephi redaction. They replace verses 46 and 47 in the Matthean text, although they were certainly not intended to parallel them. Since there were no paragraphs in the manuscript, and no verse divisions in the 1830 edition, these verses should not be seen as verse by verse replacements. Rather, they take the place of the similarly numbered verses in Matthew.
The 3 Nephi verses begin the summation of this section by reprising the statement from the beginning of this section that Jesus had come to fulfill the law (see 3 Nephi 12:17-18). “Old things” have reference to the Law, and “all things” have “become new” through the Atoning Messiah.
Referent: It is possible that the language of verse 47 owes a debt to:
2 Corinthians 5:17
17 Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.
The Matthean text: The excised verses are:
Matthew 5:46-47
46 For if ye love them which love you, what reward have ye? do not even the publicans the same?
47 And if ye salute your brethren only, what do ye more than others? do not even the publicans so?
It is probable that these verses didn’t make the transition to the New World context because of the term publican which has a clear Old World referent. They are also removed because they are the summation of the astounding introduction that claimed that the people should be more righteous than the Pharisees, which was also removed. These verses parallel that amazing statement in that Jesus once again makes a direct comparison to others. As his conclusion to how they could become more righteous than the Pharisees, something that appeared nearly impossible, is now transformed into something easier to understand. By moving past the legalities and into the true meaning of the gospel in their heart, they could become better than the Pharisees. The Pharisees loved those who loved them and saw themselves as following the Law. Jesus points out that this is only natural, and that the publicans, who were much less respected that the Pharisees, could live that simple law. Of the disciples of Christ, more was expected.