Textual: Note that Mormon records Nephi as saying “it was said by some….” This is a clear indication that Nephi is constructing his account after the fact, and that he has discussed these events with other people. As with all spectacular phenomena, there were multiple accounts and opinions. Of course the ancient world had to way to determine time, particularly when the sun was not available, so there was no way to judge the time. Nephi’s estimate was just as much a guess as were those of the people with whom he consulted.
Geological: The number three shows up in the hours and in the days. In both cases, it would have been difficult if not impossible to determine the precise duration of either measure. There were no clocks for the hours, and the darkness precluded the division of day and night. The number three shows up here for symbolic reasons. Nevertheless, a three-hour duration of the thunderings and lightnings would be consistent with an explosive eruption.