As a son of Otto, he was entitled to use the title of Margrave and he did sign some official documents, but he was never more than a co-ruler.
After his sons Otto and John died (around 1299), Albert III sold Stargard to his son-in-law Henry II of Mecklenburg.
The 1304 Treaty of Vietmannsdorf confirmed Henry II as Lord a Stargard and enshrined that it was held as a fief from Brandenburg.
In 1299, a year before his death, he founded the Cistercian monastery Coeli Porta ("Gate to Heaven") in Lychen.
Albert III died on 3 December 1300 and was initially buried at Lehnin Abbey.