Heraklios 3 was bishop of Ionopolis in Paphlagonia; in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and was probably present at all eight sessions, from 24 September to 23 November 787: Mansi XII 998-XIII 486. In the lists of those present at the first and seventh sessions he is styled Ἡρακλείου ἐπισκόπου Ἰουνοπόλεως: Mansi XII 998, XIII 369 (Ἰωνοπόλεως). Elsewhere he is styled Ἡράκλειος ὁ ὁσιώτατος ἐπίσκοπος Ἰουνοπόλεως or similar: Mansi XII 1099, XIII 145, 392. At the second session he supported the traditional veneration of icons as described in statements from pope Hadrian I (Hadrianos 1) and condemned iconoclasts: Mansi XII 1099. At the fourth session he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 145. At the seventh session he subscribed the statement of the faith adopted by the Council: Mansi XIII 392.

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