Here we have another example involving number switching in the generic pronoun. The passage begins with the plural pronouns they and their but ends up switching to the singular he. One possible reason for this is that the initial occurrences of they and their in this verse refer to the generic person (“that they might reap their rewards according to their works”) but subsequently the passage uses the plural they to refer to works (“whether they were good or whether they were bad”). In any event, the singular he was changed to they by Joseph Smith in his editing for the 1837 edition.
Number switching for generic pronouns was fairly common in the original text. For a nearby example, see Mosiah 29:28. For additional discussion, see under 1 Nephi 10:18–19. The critical text will in each of these instances maintain the reading of the earliest textual sources, thus he here in Alma 3:26.
Summary: Restore in Alma 3:26 the singular he, the reading of the earliest text; mixture of number for the generic pronoun was fairly common in the original text.