He ascended the throne when he was 30, and married Berenice II,
the daughter of the king of Cyrene, King Magus.
Following the murder of Antiochus II in Syria, Ptolemy led his army
to save his sister, who was married to Antiochus, but he couldn't
make it on time. Nevertheless, he gained victory and went as far
as Babylonia, but he was forced to return back as a result of internal
unrest. His rival, the Seleucid king, gained much of the territories
that he left behind. The Seleucid king couldn't get south Syria
from Ptolemy though.
Ptolemy III started the building of Horus
temple at Edfu, which was later completed by Ptolemy VIII.
He also added to the temple
of Karnak. Ptolemy III reigned for 25 prosperous years.