Peter, James, and John

"The event thus referred to, occurred immediately after the conversation about what men thought of Christ. Christ prepared them for it by saying, “Verily I say unto you, there be some standing here which shall not taste of death till they see the Son of Man coming in His kingdom.” The transfiguration was a vision of this, in a realistic presentment. About a week after this saying, he took Peter, James, and John—the three who were always prominent, and whom Jesus on several other occasions drew specially near to himself—the only three, also, out of the twelve, who have contributed to the apostolic writings, with the exception of Jude."  (Nazareth Revisited)

It is also interesting that the record of the transfiguration is included in the gospel accounts of Matthew, Mark, and Luke.  However, John -- the only gospel writer who witnessed the transfiguration -- did not write about it.