This verse can be confusing because the three Nephites were both transfigured and translated. They were transfigured when they were caught up into heaven and heard unspeakable things, and they were translated when a change was wrought upon their bodies whereby they would remain on the earth to bring souls to Christ. Jeffrey R. Holland said, "A person who is transfigured is one who is temporarily taken into a higher, heavenly experience, as were Peter, James, and John, and then returned to a normal telestial status." (Christ and the New Covenant, p. 306)
Translation is the process by which a mortal body of the telestial order is changed to a mortal body of the terrestrial order. The word "mortal" in this sense, means a being whose body and spirit have not been permanently united by the resurrection. (See commentary for verse 8). Richard Cowan succinctly stated, "We are telestial mortals, translated beings are terrestrial mortals, while exalted resurrected beings are celestial immortals." (Alma, The Testimony of the Word, pp. 201-2 as taken from Latter-day Commentary on the Book of Mormon compiled by K. Douglas Bassett, p. 464)
We can learn a lot about translated beings from descriptions of the three Nephites:
1. They, like John the Revelator, will 'never taste of death'
2. They will 'be changed in the twinkling of an eye from mortality to immortality' (This reaffirms that a translated being is still "mortal," for the change from mortality to immortality occurs at the Second Coming, see v. 8)
3. They would experience no pain while they dwelt in the flesh,
4. Translated beings have knowledge and wisdom given unto them that exceed human perspective.
5. Wicked or evil men and women have no power over them.
6. They are as angels, administering to whomsoever they will
7. Satan can 'have no power over them'
8. They were sanctified in the flesh that they were holy (v. 39).
9. They belong to a terrestrial order.
10. They were to remain in this translated state until the 'judgment day of Christ,' or in the words of the Savior until 'I shall come in my glory with the powers of heaven'" (adapted from Clyde J. Williams, Book of Mormon Symposium Series, 3 Nephi 9-30, edited by PR Cheesman, MS Nyman, and CD Tate, Jr., 1988, p. 241-7)
Joseph Smith
"Now the doctrine of translation is a power which belongs to this Priesthood. There are many things which belong to the powers of the Priesthood and the keys thereof, that have been kept hid from before the foundation of the world; they are hid from the wise and prudent to be revealed in the last times.
"Many have supposed that the doctrine of translation was a doctrine whereby men were taken immediately into the presence of God, and into an eternal fullness, but his is a mistaken idea. Their place of habitation is that of the terrestrial order, and a place prepared for such characters He held in reserve to be ministering angels unto many planets, and who as yet have not entered into so great a fullness as those who are resurrected from the dead. 'Others were tortured, not accepting deliverance, that they might obtain a better resurrection.' (See Hebrews 11:35.)
"Now it was evident that there was a better resurrection, or else God would not have revealed it unto Paul. Wherein then, can it be said a better resurrection. This distinction is made between the doctrine of the actual resurrection and translation: translation obtains deliverance from the tortures and sufferings of the body, but their existence will prolong as to the labors and toils of the ministry, before they can enter into so great a rest and glory." (Teachings of the Prophet Joseph Smith, p. 170-1)