In this passage, Joseph Smith edited both instances of which to a form of who (the subject who in the first case, the object whom in the second). In both cases, Joseph interpreted the which as referring to a person, in the first case to Jesus Christ, in the second probably to the servant mentioned in verse 10 who will bring these words forth (that is, Joseph Smith). This interpretation is correct in the first case, but not in the second. In the latter case, the which actually refers to the earlier my words, thus “my words ... which the Father shall cause him to bring forth unto the Gentiles and shall give unto him power that he shall bring them forth unto the Gentiles”. Of course, in the critical text both instances of which will be restored no matter whether they refer to persons or not. But the use of whom in the edited text for the second instance of which appears to be an error. The editors for the 1920 LDS edition recognized that whom was an error and thus restored the original which to the text here.
A similar error occurred later in Joseph Smith’s editing of the printer’s manuscript for another theoretically ambiguous case of which:
Here in his editing for the 1837 edition, Joseph emended the which to whom, thinking that the relative pronoun referred to “Moses my servant”; yet the which actually refers to “the law of Moses my servant”. In this instance, the change to whom was never made in the actual 1837 edition, probably because it was figured out that the which referred to the law, not Moses. Again, of course, the critical text will maintain the original which irrespective of whether it refers to persons or not.
Summary: Accept both instances of original which in 3 Nephi 21:11, the reading of the earliest textual sources; the first which refers to Jesus Christ, the second to the words that the Lord’s servant will bring forth; similarly, the which in 3 Nephi 25:4 refers to the law of Moses rather than to Moses himself.