It is a little ambiguous whether Mormon or Nephi3 wrote the majority of 3 Nephi. Scholars suggest that it was mostly written by Nephi, but Mormon broke in at a few places. Mormon was very careful to indicate when he was interjecting select commentary.
The text records that Nephi3 had several defining experiences in both his youth and in his leadership role. The early years of Nephi3 were eventful and influential. The various events that affected his training and values led him to become a great prophet at a critical time of that dispensation.
We can estimate an approximate time of birth for Nephi3 by evaluating the surrounding circumstances. Nephi’s father was a missionary that left for long periods to teach various segments of the Nephite population. It is unlikely that Nephi3 would have been born before his father returned from a seven-year mission to the land northward in the sixty-ninth year of the reign of the judges. It is more likely he would have been born around the seventieth year of the judges.
Therefore, in the ninety-first year of the judges—the year of the fulfillment of Samuel the Lamanite’s prophecy—he would have been twenty-one or twenty-two years old, depending on when his birthday fell in that year. Thirty-four years later, he would stand in the presence of the Savior at Bountiful. He would have been about fifty-six years old at that time, a little longer than the customary fifty years for a high priest.
The birth of a child is a more significant moment for a young parent than almost any other event. We are told that Nephi4, the son of Nephi3, wrote the book 4 Nephi and kept the record for 84 years. Nephi4 was possibly born somewhere in those first five years after the sign of the birth of the Savior. As a 22-year-old high priest, Nephi3 would have had to have been married. Considering the nature and knowledge of this young Nephi, his parents clearly took their responsibilities for teaching the gospel seriously.
Nephi3 was called by his father to be the high priest early in life. He is never referred to as the high priest in the record, but his priestly responsibilities and stewardships are implied or presupposed throughout this book. Since he was not the chief judge (Lachoneus), and he was not a military commander, the only thing that he could have been, in light of his duties, was the high priest. As the high priest, he was accountable for “all those things that had been kept sacred from the departure of Lehi” (3 Nephi 1:2). He was in charge of the church, and he was responsible for the temple in the city of Zarahemla where he lived. He lived in the same city in which King Benjamin had spoken. It was the city in which Alma the Younger, his ancestor, had been the first Chief Judge under the reign of the judges. His great-great grandfather and his ancestors, Alma, Helaman, Helaman, and Nephi, all had their home in the City of Zarahemla.
Book of Mormon Central, “Why Should 3 Nephi be Read as the Book of the High Priest Nephi? (3 Nephi heading),” KnoWhy 577 (September 8, 2020).