The transfiguration was a vision: Matt. 17:9: "And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead."
Moses, Elijah and Enoch all died. They were not taken to heaven, nobody is in heaven. Elijah was taken to heaven H8064. The vision showed the glorified Christ: he was still alive and not glorified.
heaven Strong's H8064
שָׁמֶה שָׁמַיִם
shâmayim shâmeh
shaw-mah'-yim, shaw-meh'
The second form being dual of an unused singular; from an unused root meaning to be lofty; the sky (as aloft; the dual perhaps alluding to the visible arch in which the clouds move, as well as to the higher ether where the celestial bodies revolve): - air, X astrologer, heaven (-s).