Notice that the three transfigured Nephites are "never to taste of death," and yet when their mission is completed we know they cannot be resurrected and exalted until they have passed through the travail of death. This must happen to all those who are transfigured. The Lord says this is necessary because the spirit must be separated from the body long enough to be quickened. Then the quickened spirit becomes the means by which the body is quickened in the resurrection. The body is resurrected to that same degree of glory to which the spirit was quickened.1 The spirit does not need to be separated from the body for a long period of time. The Lord told the three Nephites that when he came in glory they would be changed "in the twinkling of an eye from mortality to immortality."2
The three Nephites had actually ministered to Mormon, and they had also ministered to Moroni.3 Mormon says that even though he had this close association with the three Nephites he still could not tell whether they were mortal or immortal.4