As discussed under Enos 1:17, the that is optional for the phrase “knew (that) it …”, with most cases having the that, but two lacking it: nearby in 3 Nephi 4:33 (“and they knew it was because of their repentance and their humility”) and here originally in 3 Nephi 5:2 where both 𝓟 and the 1830 edition lack the that (which means that 𝓞 probably did too). In this case, the that was added in the 1837 edition, perhaps unintentionally (Joseph Smith did not insert it in his editing of 𝓟 for that edition). The source for adding the that was probably the two preceding instances of “they knew that it must (needs) be” at the beginning of this chapter:
The critical text will restore the earliest reading without the that for the third instance here in 3 Nephi 5. For more examples of this kind of variation, see the general discussion under that in volume 3.
Summary: Restore the earliest text near the end of 3 Nephi 5:2 where that is lacking after the pasttense verb form knew: “they knew it must needs be that all things should come to pass”; although for most cases in the Book of Mormon of “knew (that) it …” the that is present, there are a couple of cases where it is lacking.