In the printer’s manuscript, scribe 2 originally wrote the singular remnant, then immediately added the plural s with heavier ink flow (he apparently redipped his quill here before writing the plural s). The 1830 edition has the plural, so the original manuscript undoubtedly also did.
Normally the text has the singular remnant (61 times in the original text), but there is one other firm occurrence of the plural:
The use of the plural branches confirms the plural remnants in this second passage (“the natural branches of the olive tree or the remnants of the house of Israel”). Thus the plural remnants is possible, and in 3 Nephi 20:13 the original manuscript undoubtedly read in the plural.
It should be noted that in this passage the relative pronoun which was never grammatically emended to who. Other passages in the Book of Mormon show that for the noun remnant an original which can be edited to who(m) or that in the original text the relative pronoun can be who(m):
Here in 3 Nephi 20:13, Joseph Smith left the which unchanged in his editing for the 1837 edition. The critical text will follow each original instance of which. For further discussion of this issue, see under which in volume 3. For other examples where Joseph apparently neglected to edit which to who(m), see under 2 Nephi 18:18. Also see under 2 Nephi 25:22 for a noun (namely, nation) for which the editing from which to who(m) has been optionally applied.
Summary: The plural remnants was undoubtedly the reading of the original manuscript in 3 Nephi 20:13; this plural usage, although fairly rare in the Book of Mormon, is supported by the reading in 1 Nephi 10:14; original instances of which where who(m) is expected in modern English will be either restored or maintained, as the case may be.