The printer’s manuscript has simply “that these might come forth”, while the 1830 edition has “that these things might come forth”. Normally in the Book of Mormon text, the use of a subject these alone refers to people rather than writings or words. For instance, whenever the subject these is followed by a modal verb (like might here in 3 Nephi 21:4), the demonstrative pronoun refers to people but not to objects:
Of course, it is not impossible for these alone to refer to things, as in the following examples where these acts as a fronted direct object:
Here in 3 Nephi 21:4, our expectation is to read these things rather than just these. Thus one could argue that 𝓞 lacked the things and that the 1830 typesetter supplied it since he expected it. Even then, perhaps the original text itself had things but it was omitted during the dictation of the text.
When we turn to the evidence from the early transmission of the text, we find that there are no cases where the 1830 typesetter ever added (or omitted) the word thing, in either the singular or plural. There is one case where Oliver Cowdery may have accidentally added the singular thing (see under 2 Nephi 8:4), but there is no example where Oliver accidentally omitted either thing or things. Interestingly, however, there is one clear case where scribe 2 of 𝓟 accidentally omitted things:
In that instance, unlike here in 3 Nephi 21:4, Oliver Cowdery supplied the missing things when he proofed 𝓟 against 𝓞. Moreover, 𝓞 is extant in Alma 12:28 and has things. Notice also that in Alma 12:28 we have one more instance of these things, just like here in 3 Nephi 21:4. Thus the odds are quite high that scribe 2 of 𝓟 is responsible for the variation in 3 Nephi 21:4. The critical text will therefore accept the 1830 reading here as the reading in 𝓞.
Summary: Accept in 3 Nephi 21:4 the 1830 reading with its occurrence of things (“that these things might come forth”); there is independent evidence (in Alma 12:28) that scribe 2 of 𝓟 could miswrite these things as simply these.