As explained under 2 Nephi 25:11, the Book of Mormon text permits a clause coming after notwithstanding to be headed by the subordinate conjunction that, although in most cases the that is lacking (as in modern English). The original text has two instances of the archaic “notwithstanding that ” (in 2 Nephi 25:11 and here in 3 Nephi 16:8). In both cases, the that was removed in the editing for the 1837 edition. The critical text will restore both instances of the that. For a general discussion regarding notwithstanding in the Book of Mormon text, see under Alma 17:15.
Summary: Restore the subordinate conjunction that in 3 Nephi 16:8, the reading of the earliest text; this use of that is intended and can be found elsewhere in the original text (namely, in 2 Nephi 25:11).