Hilarion 3 was a monk of the monastery of the Hagia Theotokos of the Kellia; in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and is attested at the fourth session, on 1 October 787, when he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 156 (Ιλαρίων μοναχὸς τῆς ἁγίας Θεοτόκου τῶν Κελλίων (the text gives no breathing for Hilarion); the Latin version, at XIII 155, calls him "Hilarion monachus sanctae Dei genitricis"). He probably attended other sessions of the Council also; see Gregorios 45.

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