They were Peter, James and John.

[Luk 9:27 NKJV] 27 "But I tell you truly, there are some standing here who shall not taste death till they see the kingdom of God."

[Mat 16:28 NKJV] 28 "Assuredly, I say to you, there are some standing here who shall not taste death till they see the Son of Man coming in His kingdom."

[Mat 17:1-2 NKJV] 1 Now after six days Jesus took Peter, James, and John his brother, led them up on a high mountain by themselves; 2 and He was transfigured before them. His face shone like the sun, and His clothes became as White as the light.