Here both the 1830 edition and the printer’s manuscript have the preposition unto. The 1837 edition changed the preposition to to. This change seems to be unintentional; it was not marked by Joseph Smith in the printer’s manuscript. In four other instances, the 1837 edition accidentally changed the archaic unto to the expected to:
Clearly, the 1837 typesetter tended to replace unto with to.
There are no other occurrences in the text of advantage followed by unto or to. Either unto or to works in 3 Nephi 4:18, so the original reading will be restored in the critical text.
Summary: Restore the original preposition unto in 3 Nephi 4:18 (“this was an advantage unto the Nephites”), the reading of both 𝓟 and the 1830 edition.