Jesus’ sermon in Galilee (Matthew 5-7) was directed almost exclusively to the Twelve. Though there were no doubt other disciples present, the Sermon on the Mount was essentially an apostolic preparation address, a type of missionary training center for special witnesses (see especially the Joseph Smith Translation of Matthew 7).
The Bountiful Sermon in 3 Nephi was delivered both to the twelve Nephite Apostles and to a multitude of other faithful people. In Bountiful, whenever Jesus desired to deliver a specific message to the multitude or to the Twelve, Mormon’s account makes special notice of it (see, for example, 3 Nephi 12:1; 3 Nephi 13:25; 3 Nephi 14:1; 3 Nephi 18:26).