He took these three with him, because they had been the witnesses of his glory in his transfiguration (ch. 17:1, 2), and that would prepare them to be the witnesses of his agony.
Matthew Henry
Now the Lucan form of the transfiguration tells us that the disciples had a glimpse of his “glory” (9:32); the mention of “glory” is found in no other account of the transfiguration in the Synoptics.
