Book of Mormon scribes frequently mixed up the grammatical number for the word work (for an extensive list of examples, see under 2 Nephi 30:17). Here in 3 Nephi 21:5, we have two instances of variation involving work(s). Parallelism suggests that in this passage both occurrences of work(s) should be in the plural. For the first occurrence, scribe 2 of 𝓟 initially wrote the singular work, an obvious error because of the preceding these. But he immediately corrected the singular to the plural by adding the plural s inline; he probably redipped his pen before writing the s since it was written with heavier ink flow. However, in the second occurrence, he wrote work, which in this case was left unchanged. The first example could be interpreted as showing that scribe 2 tended to write the singular work instead of the correct plural. (For a list of other cases where scribe 2 of 𝓟 initially wrote work instead of the correct works, see under Alma 7:24.) Here in 3 Nephi 21:5, one could assume that his second work is a mistake based on this same error tendency. The 1830 edition has the plural works in both cases. The LDS text has maintained this consistent plural reading, while the more recent RLDS text (following the reading in 𝓟 since 1908) has had the singular work in the second case.
David Calabro argues (personal communication) that Jesus’s language here in verse 5 parallels what he spoke just before:
In verse 2, Jesus refers to the things that he is now declaring and will declare later (either by himself or by the Holy Ghost); in verse 5 Jesus refers to the works he is now doing and the works that will be done later (presumably by himself and by those acting in his name):
3 Nephi 21:2 3 Nephi 21:5 (the 1830 reading)
when these things which I declare unto you when these works
and which I shall declare and the works which shall be
wrought
unto you hereafter among you hereafter
The parallelism between the two verses supports having works in the plural both times in verse 5.
Don Brugger points out (personal communication) that the reference to works here in 3 Nephi 21:5 could also be referring to the future events discussed more generally in the preceding chapter, in 3 Nephi 20. Under either interpretation, the preceding text implies that the plural works is probably correct both times in 3 Nephi 21:5. The critical text will maintain each instance of works in “therefore when these works—and the works which shall be wrought among you hereafter— shall come forth”. Under this interpretation, it appears that scribe 2 of 𝓟 twice wrote works as work; he caught the first instance since it was preceded by these, which may have distracted him from correcting the second instance.
Summary: Maintain in 3 Nephi 21:5 both instances of the plural works: “therefore when these works— and the works which shall be wrought among you hereafter—shall come forth”; the plural reference to things earlier in verse 2 supports the use of the plural works in referring to what will happen later as Jesus continues to visit among the Nephites.