As they were departing (ἐν τῷ διαχωρίζεσθαι ἀυτοὺς)
Lit., in their departing. The verb only here in New Testament. The whole sentence is peculiar to Luke's narrative.
Master
See on Luk 5:5.
Let us make
See on Mat 17:4.
Tabernacles
See on Mat 17:4. “Jesus might have smiled at the naive proposal of the eager apostle that they six should dwell forever in the little succo=th of wattled boughs on the slopes of Hermon” (Farrar).
Not knowing what he said
Not implying any reproach to Peter, but merely as a mark of his bewilderment in his state of ecstasy.