As discussed under Alma 46:24, there are a few cases in the original text of the double genitive (that is, possessive phrases of the form “X of Y’s”), namely:
But there are also several cases where Oliver Cowdery initially added the possessive s to create a double genitive that was not in the original text. In each of those cases, he caught his error. (All these cases, both the original and the momentary ones, are listed under Alma 46:24.) Here in 3 Nephi 4:8, Oliver had almost finished writing the possessive Giddianhis; just as he started to write the possessive s, he aborted it and overwrote the incomplete is with an i. (Oliver typically did not supply apostrophes. For discussion of this aspect of his scribal practice, see under the phrase “three days’ journey” in the 1 Nephi preface.) In each possible case of the double genitive, the critical text will follow the earliest reading, thus “the army of Giddianhi” here in 3 Nephi 4:8.
All other instances in the text of “the army of X” (where X is a personal name) support this reading; there are no instances elsewhere in the text of “the army of X’s”, even as a scribal slip. In all, there are 25 other instances of “the army of X”, such as “the army of Moroni” (6 times), “the army of Coriantumr” (6 times), “the army of Antipus” (4 times), and “the army of Shiz” (3 times).
Summary: Maintain in 3 Nephi 4:8 the phrase “the army of Giddianhi” rather than the double genitive that Oliver Cowdery started to write in 𝓟, “the army of Giddianhi’s”; elsewhere in the text there are examples of only “the army of X”, never “the army of X’s” (where X is a personal name).