Here the 1841 British edition omitted the preposition unto. It was restored in the subsequent LDS edition (1849). Either reading is theoretically possible. Note, however, the use of unto earlier in the verse (“he commanded them that they should write the words which the Father had given unto Malachi which he should tell unto them”). For another case where the 1841 typesetter omitted unto, see under Mormon 3:4.
Summary: Maintain in 3 Nephi 24:1 the preposition unto in the relative clause “which he did tell unto them”.