According to John Welch, the book of Third Nephi ends, as many covenant texts do in the ancient Near East, with a series of warnings and admonitions. Having entered into a covenant, people need to be reminded of the seriousness of what they have done. At the end of King Benjamin's covenant ceremony (Mosiah 6) the first order of business was to appoint priests to remind people of the covenant that they had entered into, to recall to their memory the seriousness of the commitments that they had made. So again as you see in many ancient Near Eastern covenant or treaty documents, it's appropriate to end with a number of wo's--"Wo unto him that spurneth at the doings of the Lord" (3 Nephi 29:5) . . . "Wo unto him that shall deny the Christ and his works" (3 Nephi 29:5) . . . wo unto him that shall deny the revelations of the Lord" (3 Nephi 29:6) . . . "wo unto him that shall say at that day, to get gain, that there can be no miracle wrought by Jesus Christ" (3 Nephi 29:7). [John W. Welch, "Understanding the Sermon at the Temple, Zion Society," in Teachings of the Book of Mormon, Semester 4, p. 157]