The editors for the 1920 LDS edition inserted the relative pronoun who since the original text here in 3 Nephi 1:16 seems awkward and perhaps even ungrammatical (“and there were many … fell to the earth”). However, this construction is original to the Book of Mormon text, and there are still examples in the text of “there were many” followed by a subjectless finite predicate, with some intervening text but no relative pronoun:
The critical text will therefore restore the original text here in 3 Nephi 1:16.
Elsewhere in the text, there are similar existential expressions that end with a subjectless finite predicate (again, without any relative pronoun):
See under each of these passages for further examples.
Summary: Restore in 3 Nephi 1:16 the original reading without the expected relative pronoun for a sentence-final finite predicate: “and there were many which had not believed the words of the prophets fell to the earth and became as if they were dead”; similar examples of this kind of existential construction occurred fairly frequently in the original text (and most are still found in the current text).