Duchy of Saxe-Altenburg
The death of his older brother Christian (1663) made him the new heir of the Duchy of Saxe-Altenburg. When his father died in 1669, Friedrich Wilhelm III succeeded him; but, because he was only twelve years old, his maternal uncles, Elector Johann Georg II of Saxony and Duke Maurice of Saxe-Zeitz, assumed the guardianship of the new duke and the regency of the duchy.