When Alma the Younger left Melek, "He departed thence and travelled three-days’ journey on the north of the land of Melek," and came to the city called Ammonihah. A three days’ journey would likely have been about thirty to forty miles. What might be symbolized by these three days? Why did the author, and record keeper, and abridger all include this detail? They could have said, "He went from there to the city of Ammonihah," but someone thought it worth telling us that it was a journey of three days. Where else do we see three days? In 1 Nephi 2:6, Lehi traveled three days before building an altar to give thanks. In that case, being three days away from Jerusalem, Lehi was out of the temple district of the Temple in Jerusalem.
In the crucifixion and death of the Savior, very clearly, according to the Book of Mormon, there was darkness in the land of Bountiful for three days when Jesus was in the tomb (3 Nephi 8:3, 23; see also 1 Nephi 19:10; 2 Nephi 25:13; Helaman 14:20, 27).
There are other examples of three days and three nights. The sign of Jonah was mentioned by Jesus when the Pharisees demanded a sign. Jesus replied "An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas: For as Jonas was three days and three nights in the whale’s belly; so shall the Son of man be three days and three nights in the heart of the earth" (Matthew 12:39–40). There are passages in Matthew 16:4 and Luke 11:29 that refer to the sign of the prophet Jonah, but without giving its length, which was three days and three nights (Jonah 1:17). Mark 8:12 says, "There shall no sign be given unto this generation," to which the prophet Joseph Smith added "save the sign of the prophet Jonah; for as Jonah was three days and three nights in the whale’s belly, so likewise shall the Son of Man be buried in the bowels of the earth" (Mark 8:12 JST). This seems to add significance to the Savior’s prophetic pronouncement on his interment (See Cummings, "Three Days and Three Nights," 59).
Three days and three nights, then, from the time of Jonah in the mid-eighth century BC (2 Kings 14:25), symbolized a going down. "Three days and three nights" expressed a complete descent, down into death and hell; going to the bottom of the sea and into the death monster; going as far away into the darkness as possible. The mention of Alma’s three days of travel to Ammonihah may have carried the same symbolism for the Nephite record keepers. Perhaps the recorder noted this detail as he observed that "Alma went three days from Melek," which was, in other words, down, down, down, into the inferno of Ammonihah.
Further Reading
Cummings, David B. Cummings, "Three Days and Three Nights: Reassessing Jesus’s Entombment," Journal of Book of Mormon Studies 17, no. 1 (2007): 56–63, 86.