There are two problems here in this verse, both of which deal with number. The first problem concerns the word commandment(s). Lehi was given a single commandment against having more than one wife, but when Oliver Cowdery initially wrote the word in 𝓟, he wrote it as the plural commandments. He immediately corrected his error by erasing the plural s; the resulting erased s is lighter and shows minor ink smearing and some slight abrasion. The 1830 compositor correctly set the singular commandment. However, in revising the RLDS text, the editors for the 1908 edition did not recognize that the s had been erased (the s is still clearly visible), so they emended the text to read in the plural. Although 𝓞 is not extant for this particular word, the immediate correction in 𝓟 suggests that the singular is correct. Also note that the verb in the following relative clause is in the singular (“which was given unto our father”), but in truth this cannot be used as evidence to support the singular commandment since the original text allowed singular verb forms with plural subjects, as in 1 Nephi 18:15: “and loosed the bands which was upon my wrists” (also see the discussion under 1 Nephi 4:4). However, since the 1908 RLDS edition reintroduced the plural commandments, the subsequent RLDS edition (1953) changed the singular was to the plural were.
The very next verse twice refers to “this commandment”, which could be taken as support for the use of the singular commandment in Jacob 3:5 :
Of course, one could argue that these two singular instances of commandment led Oliver Cowdery to erase the plural s in the preceding verse. We also need to consider the text at the end of Jacob 2; there we have an instance of “these commandments” that refers to the same injunction against multiple wives:
One could take this reading as evidence that Jacob 3:5 should read “the commandments of the Lord”. Of course, this preceding plural commandments in Jacob 2:34 can be viewed as what led Oliver to initially write “the commandments of the Lord” in Jacob 3:5. Ultimately, the safest decision is to follow the earliest textual sources for each instance of commandment(s), which means that here in Jacob 3:5 we accept the immediately corrected reading in 𝓟 (that is, commandment). The history of the text has involved considerable variation with respect to the number of commandment. For other examples where Oliver added an s to commandment, see 1 Nephi 3:16 and 1 Nephi 4:34.
The second textual problem here in Jacob 3:5 deals with the number of father(s). The printer’s manuscript reads in the singular: “which was given unto our father”. The 1830 compositor, however, replaced this singular with the plural fathers, probably unintentionally. The 1981 LDS edition restored the singular father, but interestingly the RLDS text never has, even though 𝓟 reads father. A preceding verse in this part of the text (cited just above) clearly indicates that the commandment against more than one wife was given to Lehi: “ye know that these commandments was given to our father Lehi” (Jacob 2:34). Perhaps the RLDS editors preferred the plural use of fathers since such a reading would argue that the injunction against multiple wives predated Lehi. But as Robert F. Smith and others have pointed out, there was no injunction in ancient Israel against polygyny, so the use of the plural fathers would be problematic in Jacob 3:5; see page 79, “Textual Criticism of the Book of Mormon”, Reexploring the Book of Mormon, edited by John W. Welch (Salt Lake City and Provo, Utah: Deseret Book and FARMS, 1992), 77–79.
Summary: In accord with the earliest textual sources, maintain the singular commandment and the singular father in Jacob 3:5 (“the commandment of the Lord which was given unto our father”); the language of nearby readings supports these two singulars.