Facts and Events
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, Frederick Wilhelm III succeeded him; but, because he was only twelve years old, his maternal uncles, the Elector Johann Georg II of Saxony and the duke Maurice of Saxe-Zeitz, assumed the guardianship of the new duke and the regency of the duchy.
Only three years later, the young duke died of smallpox. With his death, the branch of Saxe-Altenburg became extinct.
The duchy of Saxe-Altenburg was split between the branches of Saxe-Gotha and Saxe-Weimar; but, on the basis of the will of the duke Johann Philipp of Saxe-Altenburg (Frederick Wilhelm III's uncle) the greater part of the duchy was retained by Saxe-Gotha, because they were the descendants of Elisabeth Sophie, the only daughter of Johann Philipp, who declared her the general heiress of the family in case of the extinction of the male line.