Agatho 3 was the author of a statement written in 713 after the overthrow of the emperor Philippikos (Philippikos 1) describing his reign and overthrow and giving some information about his own career: Agatho Diaconus, in Riedinger, pp. 898-901 (= Mansi XII 189-196).

Agatho 3 says that some thirty-two years or so before he wrote, when still a young man, he was a lector and a notarios and served at the Sixth Ecumenical Council with others, including Paulos 3 (ἔτι νέαν ἄγων τὴν ἡλικίαν, ἐν τῷ τοῦ ἀναγνώστου βαθμῷ κατηριθμημένος, ὢν καὶ ἀχρεῖος νοτάριος καθυπούργησα τῇ ἁγίᾳ ταύτῃ καὶ οἰκουμενικῇ ἕκτῃ συνόδῳ): Agatho Diac., in Riedinger, p. 898, lines 10-12 (= Mansi XI 189).

In 681 Agatho 3 was a lector and notarios of the patriarch of Constantinople: Riedinger, p. 410, line 10, p. 560, line 12, p. 614, line 16, p. 656, line 22 (= Mansi XI 461, 545, 576, 597) (Ἀγάθων ὁ εὐλαβέστατος ἀναγνώστης καὶ νοτάριος τοῦ ἁγιωτάτου ἀρχιεπισκόπου τῆς θεοφυλάκτου ταύτης καὶ βασιλίδος πόλεως or similar). At the eleventh session of the Third Council of Constantinople (the Sixth Ecumenical Council), on 20 March 681 he read out to the Council the synodika of the former archbishop of Jerusalem, Sophronios (633/634-638), addressed to the former patriarch of Constantinople, Sergios (610-641): Riedinger, p. 410 (= Mansi XI 461). At the twelfth session (22 March) he was sent with Paulos 3 and others (cf. Ioannes 32) to see Makarios 1 and to discover if certain documents brought before the Council were really his: Riedinger, p. 560 (= Mansi XI 545). He was one of those taking down (in shorthand) the proceedings of the Council: Riedinger, p. 560 (= Mansi XI 544) (see Paulos 3). At the thirteenth session (28 March) he read out the synodika of three patriarchs of Constantinople, Thomas 2, Ioannes 2 and Konstantinos 3: Riedinger, p. 614, line 17 (= Mansi XI 576). During the fourteenth session (5 April) he read out a sermon of St Athanasios which was produced in support of the doctrine of Two Wills: Riedinger, p. 656 (= Mansi XI 597). At the seventeenth session (11 September) he read out the definition of the faith prepared by the Council: Riedinger, p. 713, line 10 (= Mansi XI 624). At the eighteenth and final session (16 September) he again read out the definition of the faith, this time in the presence of the emperor and the senior lay dignitaries: Riedinger, p. 766, line 18 (= Mansi XI 632).

After the Council Agatho 3 himself in his own hand made a fair copy of all the volumes of the Acts of the Council (ἅπαντας δὲ τῶν ἐν αὐτῇ πεπραγμένων τοὺς τόμους ἐν καθαρῷ δι' ἐκκλησιαστικῶν γραμμάτων οἰκείᾳ ἔγραψα χειρί), which were then stored under seal in the imperial palace with a copy of the statement of the faith adopted by the Council; he also, on instructions from the emperor Constantine IV (Konstantinos 2), made five fair copies of the statement which were then sent to the five patriarchal sees (τοὺς ἐκδοθέντας τοῖς πέντε πατριαρχικοῖς θρόνοις ἰσοτύπους ἐνυπογράφους τοῦ αὐτοῦ ὅρου τόμους κατὰ τὸν ὅμοιον τρόπον ἔγραψα, κελευσθεὶς οὕτω παρὰ τοῦ ἐν εὐσεβεῖ τῇ μνήμῃ Κωνσταντίνου τοῦ βασιλέως); the volumes containing the Acts of the Council were subsequently burned by the emperor Philippikos 1: Agatho Diac., in Riedinger, p. 898. lines 14-15; p. 898, lines 18-20 (= Mansi XII 189, 192).

In 713 Agatho 3 was a deacon and chartophylax of the Great Church (Hagia Sophia) at Constantinople and protonotarios and second kankellarios of the patriarchal secretariate (ὁ ἁμαρτωλὸς ἐγὼ καὶ πάντων ἐλάχιστος Ἀγάθων, ἀνάξιος διάκονος καὶ χαρτοφύλαξ τῆς ἐνταῦθα ἁγιωτάτης τοῦ Θεοῦ μεγάλης ἐκκλησίας καὶ τοῦ εὐαγοῦς πατριαρχικοῦ σεκρέτου πρωτονοτάριος καὶ καγκελλάριος δεύτερος): Agatho Diac., in Riedinger, p. 898, lines 6-9 (= Mansi XII 189)). Apparently in 713 he became archidiakonos; probably in that year (see below) verses were addressed to him by the archbishop of Crete, Andreas of Crete (Andreas 3); the verses were sent when Andreas returned a work which he had borrowed from Agatho 3 and which had converted him to the orthodox view condemning monotheletism: see Heisenberg, "Ein jambisches Gedicht des Andreas von Kreta," in BZ 10 (1901), 508-512. The work was addressed: πρὸς τὸν ὁσιώτατον Ἀγάθωνα ἀρχιδιάκονον καὶ χαρτοφύλακα τῆς ἐνταῦθα ἁγιωτάτης τοῦ Θεοῦ μεγάλης ἐκκλησίας. Agatho 3 was still diakonos in 713 but later that year the monothelete cause was championed by the new emperor Philippikos 1 when he burned texts of the Council of 681; Agatho 3 was presumably promoted before that happened.

Agatho 3 is probably to be identified with the owner of a seal dateable to the early eighth century; the seal belonged to an Agatho who was protonotarios, kankellarios and chartophylax: Zacos and Veglery 1692A. Obv.: cruciform monogram of Θεοτόκε βοήθει, with the inscription: [Α - γα] - θω - νι. Rev.: .... - ....[πρ] - ωτονοτ[α] - ριω καγκ[ε] - λλαριω και χ - αρτοφυ - λακι+. He perhaps was the owner of another seal also, from an earlier stage in his career, which was owned by an Agatho who was diakonos, notarios and kankellarios: Laurent, Corpus V 2, 1613. The titles read: διακόνῳ, νοταρίῳ καὶ καγκελλαρίῳ.

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