G3801 ὁ ὢν καί ὁ ἦν καί ὁ ἐρχόμενος

the one being and the one that was and the one coming, i.e. the Eternal, as a divine epithet of Christ

which art (is, was), and (which) wast (is, was), and art (is) to come (shalt be)

a phrase combining G3588 (ὁ) with the present participle and imperfect of G1510 (εἰμί) and the present participle of G2064 (ἔρχομαι) by means of G2532 (καί);