Arsenios 3 was bishop of Horta; in 855 at Iguvium he met the envoys from Rome, Nikolaos 49 and Merkourios 3, bearing news to the emperor Louis II (Lodoïchos 1) of the election as pope of Benedict III (Benediktos 7); he persuaded them to conspire with him to support the rival claims of the priest Anastasius (Anastasios 25): Lib. Pont. 106. 6-7. When the imperial envoys, Adalbert 2 and Bernard 3, reached Horta, Arsenios 3 persuaded them also to support Anastasios 25: Lib. Pont. 106. 8. Under the successor of Benedict, pope Nicolas I (Nikolaos 28), Arsenios 3 was sent to Gaul to restore bishop Rothad 1 to his see at Soissons, to dissolve the marriage of king Lothar (Lotharios 2) to Waldrada 1, to restore peace and harmony between the kings in Gaul and to introduce necessary reforms in the church; he is styled a bishop and apocrisiarius and missus of the apostolic see ("Arsenium episcopum, huius almae sedis apostolicae apocrisarium et missum"): Lib. Pont. 107. 63. This mission is recorded in the Annals of Hincmar s.a. 865 (PL 125, 1221) and in papal letters (Jaffé 2773-2779, 2787, 2788, 2790-2795). Arsenios 3 apparently persuaded Nikolaos 28's successor, pope Hadrian II (Hadrianos 8), to add words in praise of Louis II (Lodoïchos 1) to a letter written by Nikoloas 28: Lib. Pont. 108. 42. Arsenios 3 was therefore still alive in 867.

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