As discussed under 2 Nephi 10:6, the original text has examples of both singular pestilence and plural pestilences. In each instance, the critical text will follow the earliest textual sources. Here in Helaman 12:3, 𝓟 and all the early editions had the plural, but the 1905 LDS edition changed the plural to the singular. The subsequent LDS text has continued with the singular, but the critical text will restore the plural.
Summary: Restore in Helaman 12:3 the plural pestilences, the reading in 𝓟 (the earliest extant source).