Dominikos 3 was bishop of Trevi (Dominicum Trivensem, sc. episcopum); he was ordered by the bishop of Rome, Nicholas I (Nikolaos 28), to go to Bulgaria with Grimoald 5, the bishop of Polimartium, and a number of priests, in order to select one of them as the new archbishop of Bulgaria; Nikolaos 28 ordered him then to proceed to Constantinople with Formosus, the bishop of Portus (Formosus 1), in connection with the schism involving the patriarchs Photios 1 and Ignatios 1; at this point the emperor Michael III (Michael 11) died and was succeeded by Basil I (Basilios 7), and Photios 1 was expelled and replaced by Ignatios 1: Lib. Pont. 107. 74-76. The date was therefore 867. The death of pope Nikolaos 28 himself delayed their departure but they were sent to carry out Nikolaos 28's instructions by the next bishop of Rome, Hadrian II (Hadrianos 8): Lib. Pont. 108. 12. With his fellow-bishops Paulos 57, Leopardus 1 and Formosus 1 he founded churches and ordained clergy for the Bulgars; by c. 870 he was apparently no longer in Bulgaria: Lib. Pont. 108. 54. He apparently returned with Leopardus 1 when the Bulgars, having requested an archbishop and been sent Silvester (PBE) by pope Hadrianos 8, sent him back with a request for Formosus 1: Lib. Pont. 108. 62.

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