[Coin depicting Arsinoë III, source: Wikimedia Commons]
Arsinoë and her brother were the children of Ptolemy III and Berenike II. Given the poets, priests and intellectuals her mother chose to surround herself with, it would be surprising if she weren’t highly educated. Her brother took the throne in 222, beginning his reign by having Berenike murdered.** He fought the battle of Raphia against the Seleukids in Palestine in 217 with Arsinoë at his side. Whether or not she actually fought is uncertain, but she was certainly there.
We also don’t know if she was married to him at this point. On the one hand, even once they married she was still his sister in addition to being his wife and such a marriage would give both of them legitimacy as rulers, stabilizing Ptolemy’s position in the early years of his reign. On the other hand, several accounts of the battle name Arsinoë his sister, not his wife. Regardless, they have to have been married sometime before 215 or so, when the pair of them were deified and incorporated into the dynastic cult. Their son and only known child Ptolemy V was born five years later in 210.
The pair of them did not get along very well and disagreed about how the Egypt should be ruled. Ptolemy loved extravagant parties and had several favorites he liked to reward with wealth and influence. Arsinoë disapproved of his wild expenditures and free handing out of power, perhaps understanding that it would lead to a weakening of their centralized government. The two never reconciled and Arsinoë had retreated from government by early 204. Ptolemy died of unknown sometime that spring or summer. Arsinoë was murdered late in the summer that same year, either on Ptolemy’s orders, if he was still alive, or by his supporters after his own death.*** Her popularity was such that a mob gathered in reaction and executed some of Ptolemy’s favorites.
*Justin calls her Eurydice. Since she isn’t referred to this way by any other ancient historian and the name had really no significance to the Ptolemaic dynasty at all, there’s really no reason to assume that was actually one of her names.
**Which makes his title rather ironic. Father, yes, mother, no.
***We don’t actually know who died first.
Polybius 5.83-84 - Lacus Curtius
Polybius 15.25 - Lacus Curtius
Justin, Historiarum Philippicarum libri 30.1-2 - Forum Romanum
3 Maccabees 1-4 - University of Michigan
Arsinoë III - Egyptian Royal Genealogy
Portrait of a Ptolemaic Queen, possibly Arsinoë III - Mount Holyoke College Art Museum
Arsinoe III - Encyclopedia Britannica