Theoktiste 3 came of a wealthy and noble family: Vita B Theod. Stud., 236C (εὐσεβεῖς καὶ εὐπατρίδαι), Vita C Theod. Stud. ¤4, p. 259 (τὸ γένος ἐπίσημοι - of Theodoros 15's parents), Vita A Theod. Stud., 116B (Πατέρες δὲ λαμπροὶ τὸ γένος, καὶ πλούτῃ μυρίῳ κομῶντες).

Theoktiste 3 was the sister of Platon 1: Vita B Theod. Stud., 240B (ἀοιδίμου Θεοκτίστης ὁμαίμων Πλάτων), Vita C Theod. Stud. ¤8, pp. 261-262 (Θεόδωρον, ἐξ ἀδελφῆς ὄντα ἀνεψιόν, καὶ μητέρα τούτου τὴν καὶ ἀδελφὴν), Vita A Theod. Stud., 121A (Θεόδωρον, ἐξ ἀδελφῆς ὄντα ἀνεψιὸν). Her parents were therefore Sergios 100 and Euphemia 4; cf. Theod. Stud., Laudatio Platonis I 3 (805A). She had three other brothers (Anonymi 31): Vita A Theod. Stud., 121A (ὁ Πλάτων καὶ ἀδελφοὺς οἰκείους ... τρεῖς). She also had a sister, Anna 13.

Her parents died when Theoktiste 3 was young and she only acquired an education after her marriage, devoting herself to the study of the Psalms when she could find time: Theod. Stud., Laud. Matris, 885B (ἀγράμματος ἐξ ὀρφανίας ἀγομένη, γραμματίζει ἑαυτὴν ἡ σοφὴ καὶ συνετίζει, καὶ τὸ Ψαλτήριον ἀποστηθίζει κάλλιστά τε καὶ συντομώτατα). She was wife of Photeinos 21 and the mother of Theodoros 15 (Theodore the Stoudite), who was born in Constantinople in c. 759: Vita B Theod. Stud., 236C-237A (καὶ αὐτὴ δὲ ἡ μήτηρ ὡς περιφανὴς καὶ ἐπίδοξος, πυκνότητι πυκαζομένη καὶ καλῶς εἰδυῖα τῶν τε ἰδίων τέκνων καὶ τοῦ οἴκου προΐστασθαι, ἅτε ὑπὸ Θεοῦ κτισθεῖσα, καὶ τὴν ὁμοίαν προσηγορίαν τοῖς ἔργοις σφραγισθεῖσα. τὸ γὰρ κοινὸν τῆς θεοκτισίας ὄνομα ἰδιάζον αὐτῇ παρυπάρξαν διὰ τῆς κατ' ἀρετὴν εὐζωϊας, Θεοκτίστη καὶ ἦν καὶ ὠνόμαστο, τὸν ἐξ ἀμφοῖν ἔπαινον διὰ τῆς σεμνῆς κερδαίνουσα πολιτείας), Vita C Theod. Stud. ¤4, p. 259 (πατέρες δὲ Φωτεινὸς καὶ Θεοκτίστη), Vita A Theod. Stud., 116BC (Ὄνομα τῷ μὲν τῶν γονέων Φωτεινὸς, τῇ δὲ Θεοκτίστῃ), 121A.

Theoktiste 3 had two other sons (Ioseph 3 and Euthymios 14) and a daughter (Anonyma 74): Vita C Theod. Stud. ¤8, pp. 261-262 (σὺν ἅμα δυσὶ τοῦ Θεοδώρου ἀδελφοῖς καὶ μικρᾷ ἀδελφῇ), Vita A Theod. Stud., 121A (πατέρας τοῦ Θεοδώρου καὶ ἀδελφοὺς, Ἰωσὴφ καὶ Εὐθύμιον, σὺν ἅμα καὶ μικρᾷ ἀδελφῇ), cf. Theod. Stud., Ep. 1 (she had three sons), Ep. 6 (mother of Theodoros 15, Euthymios 14 and a daughter, Anonyma 74), Theod. Stud., Laud. Matris, 888A (her daughter).

After the birth of their children, she and her husband lived for five years in continence: Theod. Stud., Laud. Matris, 885D-888D. She was devoted to the veneration of icons, like Photeinos 21, and she became a nun at the same time as he became a monk: Vita A Theod. Stud., 116C-117A (τὸν ἀναχωρητικὸν εἰτ' οὖν μοναδικὸν βίον ἀσπάζονται). She like her husband followed the life of a hermit with distinction (τὰ ὅμοιά τε τῷ ἀνδρὶ δέδρακε καὶ τοῖς ἀσκητικοῖς ἀγῶσι τὸ θῆλυ τῆς φύσεως ἀπηρρένωσεν): Vita C Theod. Stud. ¤4, p. 259. She was eventually persuaded by Platon 1 to sell up the family property and with Theodoros 15 and other members of the family to retreat to Bithynia, where she and her daughter became nuns in a convent at Sakkoudion, close to the monastery there under Platon 1 and Theodoros 15: Vita A Theod. Stud., 121A, Vita B Theod. Stud., 240D-241B (μάλιστά γε μὴν τὴν ἰδίαν ἀδελφὴν, sc. of Platon 1), Vita C Theod. Stud. ¤¤8-9, pp. 261-262. Influenced by her brother, she persuaded her husband, her four children and her husband's three brothers to abandon the world and follow a religious life, thereby giving up a comfortable life and a position in the imperial service (she is described as ἀξιώματι βασιλικῷ καὶ ταμιευτικῷ τιμωμένη): Theod. Stud., Laudatio Matris, 889BC. The date was either c. 781 (soon after the accession of Eirene 1) or else in or soon after 787 (after the Council of Nikaia); cf. Platon 1.

She was the addressee of a letter from Theodoros 15, written when she was gravely ill and old (ἐν γήρει καλῷ); she is addressed as τιμία καὶ γλυκεῖα καὶ θεοπόθητέ μου μῆτερ; her daughter and her son Euthymios 14 were both already dead: Theod. Stud., Ep. 6 (addressed Θεοκτίστῃ τῇ ἑαυτοῦ μητρί). Her death is mentioned by Theodoros 15 in a letter to Anna 12: Theod. Stud., Ep. 42. She is mentioned in two other letters of Theodoros 15: Theod. Stud., Ep. 44, 85. Theodoros 15 composed a funeral sermon on her: Theod. Stud., Laudatio Matris (PG 99, 884-901).

Theoktiste 3 opposed the second marriage of the emperor Constantine VI (Konstaninos 8): Theod. Stud., Ep. 1, Theod. Stud., Laudatio Matris, 893Aff. She was at Paula with Sabas 4 when Theodoros 15 passed through there on his way to exile at Thessalonike (in c. 796); they were able to meet secretly and spent the night in conversation: Theod. Stud., Ep. 3, Theod. Stud., Laudatio Matris, 896A. She gave help and comfort to the monks who resisted the emperor and herself suffered persecution in consequence: Theod. Stud., Laudatio Matris, 896Bff.

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