Here the 1840 edition omitted the preposition unto, probably unintentionally. This shorter reading has been maintained in the RLDS textual tradition. The LDS text, deriving from the 1837 edition (through the 1841 British edition), has retained the unto. As explained nearby under 3 Nephi 12:19, the Book of Mormon text allows for either reading, with or without the unto, in expressions like this. The critical text will maintain the unto here since it is the earliest extant reading. In this instance, there is no corresponding reading in the King James version of the Sermon on the Mount.
Summary: Maintain in 3 Nephi 12:29 the preposition unto, the reading of the earliest text: “I give unto you a commandment”.