The 1830 edition and the printer’s manuscript agree here in verse 18 by having great modify commander. But there is disagreement over whether the plural word captains is preceded by chief. The use of chiefest along with chief captains (the reading in 𝓟) seems somewhat odd, which may have led the 1830 typesetter to drop the chief in front of captains (giving “the chiefest among all the captains”). One could argue that Oliver Cowdery accidentally added chief before captains because of the preceding chief captains in verse 17 (listed above as 1). On the other hand, one could argue that chief captains is correct because it is otherwise used throughout this passage (not only in verse 17 but also in verse 19, listed above as 3). The longer expression (“the chiefest among all the chief captains”) was restored in the 1908 RLDS edition and in the 1981 LDS edition, in both cases under the incorrect assumption that for this part of the text the 1830 edition derived from 𝓟 rather than from 𝓞.
The original manuscript is not extant here, but spacing between extant fragments suggests that Oliver Cowdery initially wrote in 𝓞 a text shorter by one word (probably with either chief or great initially missing). The fact that the 1830 edition is missing the word chief suggests that the original manuscript probably had chief inline and that the word great was supralinearly inserted. Usually if 𝓞 has a supralinearly inserted word, that word is faithfully copied into 𝓟 (and into the 1830 edition for this part of the text from Helaman 13 through Mormon 9). If chief had been initially missed in 𝓞 and then supralinearly inserted, the 1830 edition would probably not have dropped it. Moreover, the term great commander does seem odd for modern English readers, and there is an example in his copywork where Oliver permanently omitted the word great before a title:
There are also a few cases in 𝓟 where Oliver initially omitted great in his copywork:
So it is quite possible that Oliver dropped the great here in 3 Nephi 3:18 when he initially wrote down the text in 𝓞. In any event, the expression “the great commander of all the armies of the Nephites” is acceptable (see the discussion under Alma 58:4).
Even though one might think that there could be only one chief captain at a time, Book of Mormon usage clearly indicates that there were many chief captains at a time. In fact, the plural chief captains occurs more frequently in the text than the singular chief captain (in the original text, there are 22 instances of the plural, 8 of the singular). Most occurrences of the singular chief captain refer to the commander of the whole army:
Mormon, realizing the possible confusion in terminology for the last of these examples, distinguished between chief captains in general and the chief captain who led the whole Lamanite army by adding a corrective or-phrase at the end of the sentence:
In the two other singular cases, chief captain refers to a local commander:
In fact, there is a whole hierarchy of captains:
In all, there are four types of captains in the Book of Mormon text: captains, higher captains, chief captains, and the chief captain (or chief commander) over the whole army.
Summary: Maintain in 3 Nephi 3:18 the reading of the printer’s manuscript, “the chiefest among all the chief captains”; there is a hierarchy of captains, so that referring to the chiefest of the chief captains is not a redundancy.