Generally speaking, the current Book of Mormon text uses the plural skins to refer to the skin color of peoples:
There are two exceptions in the current text:
The first of these two instances of skin is clearly singular since it is preceded by the indefinite article a. It is possible that the second one is an error. The original manuscript is no longer extant for 3 Nephi 2:15. But in 3 Nephi, both 𝓟 and the 1830 edition are firsthand copies of 𝓞, and for this verse both read “their skin”; thus the original manuscript probably read in the singular as skin. Counting the example here in Jacob 3:9, we appear to have three occurrences of the singular skin, of which one (“a skin” in 2 Nephi 5:21) is firm. Thus the singular skin is possible. For this reason, the critical text will restore the singular skin in Jacob 3:9, even though it could be an early transmission error for skins. The 1830 compositor’s decision to set “their skins” in verse 9 was not based on the original manuscript (which he did not examine here) but instead was based on the fact that the immediately preceding text used the plural “their skins” twice (in verses 5 and 8, cited above).
The Book of Mormon text has other examples of plural uses of skins, as in discussions about how the skin of more than one person might be injured:
The second example provides independent evidence that Oliver Cowdery had problems with the plural s in skins. In the original manuscript for Alma 44:18, Oliver first wrote skin, then inserted an s after skin to form the plural. (There is no apparent difference in the level of ink flow for the inserted s, so the insertion probably occurred quite soon after writing skin.) This correction suggests that Oliver’s tendency was simply to write the singular skin —as all speakers of modern English would tend to do, even when the context applies to more than one individual.
Summary: Restore the singular skin in Jacob 3:9, even though the two preceding references to the skin of the Lamanites uses the plural skins; two other passages support the use of the singular skin to refer to more than one person: 2 Nephi 5:21 (“a skin”) and 3 Nephi 2:15 (“their skin”).