Leontios 3 was the bishop of Helenopolis in Bithynia; in 681 he attended the final session of the Third Council of Constantinople (the Sixth Ecumenical Council) (on 16 September): Riedinger II 2. 760-829 (= Mansi XI 628-680). In the subscriptions to the statement of the faith, which condemned monotheletism, he is styled Λεόντιος ἐλέει Θεοῦ ἐπίσκοπος τῆς Ἑλενουπολιτῶν πόλεως τῆς Βιθυνῶν ἐπαρχίας: Riedinger II 2. 796, line 10 (= Mansi XI 653). The old Latin version printed in Riedinger calls his see "Helenopolitanae ciuitatis Bithiniae regionis": Riedinger II 2. 797, line 11. However, the version printed in Mansi mistakenly calls his see "Helenopolitanae civitatis Trapezuntiorum": Mansi XI 654. The word "Trapezuntiorum" is repeated in error from the preceding signature, Theodoros 45 of Trapezous. In the signatures to the Council Leontios 3 is styled Λεόντιος ἐλέει Θεοῦ ἐπίσκοπος Ἑλενουπόλεως Βιθυνίας: Riedinger II 2. 829, line 2 (= Mansi XI 680). In the list of those attending the session he is styled Λεοντίου ἐπισκόπου Ἑλενουπόλεως: Riedinger II 2. 760, line 17 (= Mansi XI 628).

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