The 1841 British edition changed the plural “there were much contentions” to the singular “there was much contention”, probably under the influence of the singular much, yet in opposition to the following conjoined plural, “and many dissensions”. The RLDS text has maintained the original plural phraseology. (The 1849 LDS edition restored the plural contentions but maintained the 1841 was; this may represent a typo rather than conscious editing. The 1852 LDS edition has “there was much contention”, the 1841 reading.) For another example of the phraseology “much contentions and many dissensions” and its editing, see under the Words of Mormon 1:16. For the general possibility of much modifying plural nouns, see under Enos 1:21.
A simpler grammatical emendation here in Helaman 3:3 would have been to change much to many. And actually there are quite a few examples of conjoined noun phrases where many is repeated:
There would really be nothing wrong with editing much to many rather than changing were to was and contentions to contention. But the earliest reading, with its mixed usage, is also textually possible and will be restored in the critical text.
Summary: Restore in Helaman 3:3 the original plural noun contentions along with its plural verb were: “there were much contentions and many dissensions”; such usage exists elsewhere in the text.