Platon 3 was bishop of Ankyra in Galatia; in 681 he attended the Third Council of Constantinople (the Sixth Ecumenical Council) and was present at each of the last nine sessions (ten to eighteen, from 18 March to 16 September 681): Riedinger II 2. 280-823 (= Mansi XI 389-669). In the subscriptions to the statement of the faith, which condemned monotheletism, and to the Council, he is styled Πλάτων ἐλέει Θεοῦ ἐπίσκοπος τῆς Ἀγκυρανῶν μητροπόλεως τῆς πρώτης τῶν Γαλατῶν ἐπαρχίας: Riedinger II 2. 780, lines 13-14, 823, lines 9-10 (= Mansi XI 641, 669) (the old Latin version printed in Riedinger calls his see "Ancire metropoleos primae Galatiae regionis": Riedinger II 2. 781, line 13 (= Mansi XI 642, 670)), cf. Riedinger II 2. 892, lines 1-2 (= Mansi XI 689). In the lists of those attending each session he is styled Πλάτωνος ἐπισκόπου Ἀγκύρας Γαλατίας: Riedinger II 2. 280, line 15, 402, line 24, etc. (= Mansi XI 389, 457, etc.). He was one of those who signed the copy of the definition of the faith sent to the bishop of Rome, Agatho 1, signing on behalf of the bishops under him as well as himself: Riedinger II 2. 892, lines 1-2 (= Mansi XI 689) (ὑπὲρ ἐμαυτοῦ καὶ τῆς ὑπ'ἐμὲ συνόδου).

Possibly identical with Platon, metropolitan bishop of an unnamed see and owner of a seal dateable to the seventh century: Laurent, Corpus V 1, 944. Obv.: two bearded busts facing one another. Rev.: +Πλα - [τ]ωνος μ - [η]τροπο - λιτου.

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