Lyle Fletcher has suggested (personal communication, 23 August 2006) that the of here in “the people of Nephi did gain some advantage of the robbers” might be an error for over. Perhaps the use of of was prompted by the phrase “to take advantage of someone”, which does occur in the text:
Here in 3 Nephi 2:17, both 𝓟 and the 1830 edition agree, so 𝓞 undoubtedly had the of in “did gain some advantage of the robbers”. When we consider all other verbs in the text that have advantage as their complement, we find that over is the preposition that follows advantage, including four more with the verb gain; in fact, one is nearby in this same chapter of 3 Nephi (marked below with an asterisk):
Moreover, there are a few cases in 𝓞 where Oliver Cowdery initially wrote of instead of the correct over (but these he immediately corrected to over):
So the of in 3 Nephi 2:17 could be an error for over, at least in theory.
Of course, in current English we do have the phrase “you have the advantage of me”, so the of should not be ruled out here in 3 Nephi 2:17. More significantly, the Oxford English Dictionary states under definition 1b for the noun advantage that the preposition can be either of or over for expressions of the form “to have, gain, get, give advantage of, over” (in fact, in Early Modern English there were also examples with the preposition on). The OED cites two instances of “advantage of ” (both in Early Modern English); in each case, we would expect “advantage over” in modern English:
(Except for the bolding of the of, I cite these with their original accidentals as given in the OED.) Thus the use here in 3 Nephi 2:17 of the preposition of is acceptable, even though this usage appears to be archaic. The critical text will maintain the of.
Summary: Maintain in 3 Nephi 2:17 the preposition of in the phrase “did gain some advantage of the robbers”.