Demetrios 3 was deacon and skeuophylax of the Great Church of Constantinople (Hagia Sophia); in 787 he was one of the patriarchal officials present at the Second Council of Nikaia (the Seventh Ecumenical Council); he is styled Δημήτριος ὁ θεοφιλέστατος διάκονος καὶ σκευοφύλαξ τῆς κατὰ Κωνσταντινουπόλιν ἁγιωτάτης μεγάλης ἐκκλησίας or similar: Mansi XII 1114, XIII 8, 72, 164, 180, 184, 185. He is attested at the third session, on 28 September 787, when he announced that the former iconoclast bishops who had read their statements of faith at previous sessions were waiting at the doors ready to enter to hear the judgments of the Council on them: Mansi XII 1114. At the fourth session, on 1 October 787, he was one of the readers and read to the Council passages from St John Chrysostom and St Basil: Mansi XIII 8 (John Chrysostom), 72 (Basil). At the fifth session, on 4 October 787, he read an extract from a work by archbishop John I of Thessalonike [died c. 630 or 649; excluded from PBE] (Beck, Kirche, p. 458), from the Ecclesiastical History of Theodorus Lector and from a work on the Martyrs by the deacon and chartophylax Konstantinos 76: Mansi XIII 164 (Archbishop John I of Thessalonike), 180 (Theodorus Lector), 185-188 (Konstantinos 76). At this session he told the Council that when he was appointed chartophylax in the Great Church at Constantinople he examined the inventory of books and found that two containing silver icons were missing and on enquiring further learned that the heretics had burned them; he also found another book by the chartophylax Konstantinos 76 containing material about icons from which pages dealing with icons had been cut; he held the book up for the assembly to see, and later read extracts from it: Mansi XIII 184. See also Leontios 9.

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