It appears that all the conjuncts in this passage read in the plural. The 1830 edition gives such a consistent reading. The printer’s manuscript, on the other hand, has the singular priestcraft instead of the plural priestcrafts. In addition, scribe 2 of 𝓟 initially wrote strife, but then later (with somewhat heavier ink flow) he corrected the singular strife to strifes. This change shows that scribe 2 tended to write the singular even though his copytext apparently read in the plural. Here in 3 Nephi 21:19, the plural strifes is very likely the correct reading. For further examples of mixups between strife and strifes (including at least one more by scribe 2 of 𝓟), see under Alma 1:32. The case regarding priestcraft(s) is more complicated since 𝓟 reads in the singular and the
1830 edition is in the plural. But in all other cases where priestcraft(s) is conjoined with other nouns, all the conjuncts, where possible, are in the plural, including priestcraft(s):
Notice that in the last example scribe 2 of 𝓟 once more initially wrote strife instead of the correct strifes. The consistency of the plural priestcrafts with all the other plural conjuncts argues that in 3 Nephi 21:19 the original text read in the plural (and probably 𝓞 did too). Scribe 2 of 𝓟 seems to have accidentally omitted the plural s when he copied priestcrafts from 𝓞 into 𝓟.
Summary: Accept in 3 Nephi 21:19 the plural uses of strifes and priestcrafts, especially since in all other cases of conjunctiveness we get plural forms for these nouns and the nouns conjoined with them (as in especially the two other cases of extended conjunctiveness in 3 Nephi 16:10 and 3 Nephi 30:2).