Upon the death of his father, Welf III succeeded him in the family's extended estates in Swabia and Bavaria.

Through the intervention of his aunt, Richlind of Altdorf, he also inherited the property of her late husband Count Adalbero II of Ebersberg in 1045.

[4] Together with Duke Bretislaus I of Bohemia, Welf accompanied Emperor Henry III on his 1051 campaign against King Andrew I of Hungary.

[5] Later however, he turned against the emperor and got involved in a conspiracy around the deposed Duke Conrad I of Bavaria, who with Hungarian support raided the Carinthian March of Styria.

In 1057 Emperor Henry's widow Agnes of Poitou ceded the Carinthian duchy to the Ezzonid scion Conrad III.