However, after Jan II's death in 1376, his son could succeed him, but under the guardianship of Przemysław I Noszak.

This is confirmed by a document from 25 November 1377 in which the Duke of Cieszyn approves the Oświęcim succession.

13 May 1400), daughter of Algirdas, Grand Duke of Lithuania and sister of King Władysław II Jagiełło of Poland,[4][better source needed] which enabled him to improve the relations between Poland and his relative Władysław Opolczyk.

In 1397, together with the other Silesian Dukes and the Bishop of Wrocław, Jan III signed in Legnica an arrangement with the Polish King, to guarantee the common action against robbery.

In the internal politics, Jan III promoted and supported the development of the cities (granting privileges, among others, to the districts of Kęty and Zator) and the Church (in particular, like his father and grandfather, to the Dominicans).