The question here concerns the antecedent of the relative pronoun which: namely, does it include “the famine”? And if not, is “the pestilence and destruction” acting as a singular or as a plural? The 1907 LDS vest-pocket edition and the 1953 RLDS edition changed the original singular verb form has to the plural have, probably because the editors (or typesetters) of these editions viewed the antecedent as plural (irrespective of whether it included famine).
Note that here in Helaman 11:15 there is no repeated the before destruction. This same nonparallel conjunctiveness is found in Helaman 10:6 :
In this earlier passage, the preposition with is not repeated before destruction, but it is found before both famine and pestilence; the definite article the patterns the same way in Helaman 11:15. These two passages thus support the idea that “pestilence and destruction” can be considered a unit—that is, as a singular.
On the other hand, the context here in Helaman 11 implies that “the famine” as well as “the pestilence and destruction” is something that “has come unto them”, which may mean that we have a plural sense here in verse 15 even if “pestilence and destruction” is acting as a unit. Note the following example where famine is conjoined with pestilence and destruction:
This passage refers to destructions, famines, and pestilences all coming upon these people—that is, we have the same nominal and verbal phraseology: the three nouns famine, pestilence, and destruction (although the grammatical number is different), plus the verbal expression “come upon someone”, equivalent to “come unto someone”. This parallelism argues that the intent in Helaman 11:15 is to say that “the famine” and “the pestilence and destruction” had come upon the Nephites, not just “the pestilence and destruction”. The repeated the implies that we have a plural antecedent for the relative pronoun which. Therefore, from a grammatical point of view, the has of the current LDS text could be edited to have. The critical text, of course, allows has to serve as the verb form for plural subjects; for a list of examples, see under Mosiah 7:20. For other cases where there is a question of whether a conjunctive subject is to be considered singular or plural, see under Alma 29:5 (for “good and evil”) and Helaman 3:32 (for “peace and great joy”).
Summary: The antecedent for the relative pronoun which in Helaman 11:15 is probably the whole conjoined noun phrase (“the famine and the pestilence and destruction”); the repeated the implies that this noun phrase is plural, and therefore the singular verb form has should probably be edited to have in the standard text; the critical text will, of course, maintain the earliest reading with the has.