Here is a clause where the word girded might have originally occurred: “and they had a lambskin girded about their loins”, as in other cases in the text where girded could be removed without much difference in meaning:
Since both 𝓟 and the 1830 lack the word girded here in 3 Nephi 4:7, then undoubtedly 𝓞 also lacked it. So if the original text had girded here, it must have been lost as Joseph Smith dictated the text to Oliver Cowdery. Yet there may be a good reason for why girded is missing here in 3 Nephi 4:7: namely, the immediately preceding clause has the word: “and they were girded about after the manner of robbers”. Moreover, the reading “they had a lambskin about their loins” is possible, so it looks like what we have in the Book of Mormon text is a case of variation, three times with girded and once without. The critical text will maintain the earliest extant reading here in 3 Nephi 4:7 since it will work without girded.
Summary: Maintain 3 Nephi 4:7 without the word girded since neither 𝓟 nor the 1830 edition have it; presumably 𝓞 did not have girded either, nor is it required; perhaps girded is lacking here because the preceding clause already has girded.