He was the archiminister (archminister) and consiliarius (counsellor) of the Emperor Louis II.
In 869-870 he travelled to Constantinople as imperial missus together with Anastasius the Librarian, to negotiate a peace with the Byzantines.
He was a member of the Supponid family and was related to Louis's empress, Engelberga, and also to Suppo, count of Parma, Asti, and Turin, his cousin.
After Louis's death, at first he supported Carloman of Bavaria for the Italian throne.
After Charles the Bald obtained the crown, he pacified with him but in February 876 he was stripped of the duchy of Spoleto, in favour of the previous duke, Lambert I. Suppo III died between March 877, when he is mentioned in a document, and August 879, when a letter from Pope John VIII laments his death.