The printer’s manuscript has “I have set an example before you”, while the 1830 edition has “I have set an example for you”. In his editing for the 1837 edition, Joseph Smith altered the printer’s manuscript so that it would agree with the reading of the 1830 edition (“for you”).
Elsewhere in the text, when the article for example is the indefinite a or an, the verb phrase “set an example” takes the preposition for (just like in the 1830 reading here in 3 Nephi 18:16):
On the other hand, when the article for example is the definite the, the verb phrase “set the example” takes the prepositional before:
(See under Jacob 3:10 regarding the use of dialectal sat rather than standard set as the original verb form in that passage.) This minor correlation in usage could be taken as evidence for the preposition for in 3 Nephi 18:16. On the other hand, it should be noted that in the one case where the reference is to the Lord setting the example (in 2 Nephi 31:9), the preposition is before, thus supporting the use of before here in 3 Nephi 18:16. Internal evidence can therefore be used to argue for either preposition.
In theory, then, either before or for is possible in 3 Nephi 18:16. Yet there are no other examples of mix-ups between before and for in the history of the text, so there is no evidence from errors in transmission concerning which preposition is correct here. However, for speakers of modern English, for is the expected preposition in the phrase “to set an/the example for someone”, which suggests that the 1830 typesetter accidentally replaced the unexpected before in the original manuscript with the more expected for. It also seems less likely that an original for would have been replaced by the longer and unexpected before. Thus the odds are that the more likely reading for 𝓞 was the reading in 𝓟, “I have set an example before you”. The critical text will therefore accept the preposition before in this passage.
Summary: Restore in 3 Nephi 18:16 the less expected, but acceptable, reading of the printer’s manuscript, with its preposition before (“I have set an example before you”).