Mithridates III was a son of Mithridates II, king of Pontus, and a Seleucid princess who is usually called "Laodice". He had two sisters; one Laodice was married to a Seleucid prince named Achaeus, another Laodice was married in 222 BCE to the future Seleucid king Antiochus III the Great.
The reign of Mithridates III appears to have started (or the reign of his father ended) with an unsuccessful attempt to take Sinope, one of the Greek ports on the southern shore of the Black Sea [Polybius, World History 4.56.] but little else is known, except for the fact that Amasia was his residence and that he was the first Pontic king to mint coins with his own portrait.
Mithridates III was succeeded by his son Pharnaces I, who captured Sinope. 1
By 220 BC, the Mithridatids had worked out a concrete plan to consolidate their position on the coast of the Black Sea and to capture the greatest Hellenic coastal city, Sinope. In 220 BC Mithridates III besieged Sinope, which was backed by Rhodes, Cos and some other Greek cities. All of Sinope's supporters had commercial interests in the Euxine, and their assistance turned out to be decisive. The siege failed (Polyb. IV. 56), but the Pontic kingdom received diplomatic support and secured their rear in the south. The alliance with the Seleucids ... could assist in aggressive plans on the northern Anatolian coast. The dynastic marriages with Antiochus III and Achaeus were favourable for Pontus because they gave it strong allies for future territorial acquisitions and helped to achieve agreement about returning ancestral domains in Phrygia and Mysia. That was a new advantage for Pontic rulers who were still hoping to strengthen links with the Seleucids.
Unfortunately, the Mithridatids did not achieve their aims concerning Phrygia. Neither Achaeus nor Antiochus III fulfilled the promise of Seleucus II Callinicus to cede Phrygia to the Kingdom of Pontus. In the treaty of Apamea of 188 BC, when the Romans had enlarged the territorial possessions of their ally Pergamum with the lands "on this side of Mt Taurus" including the greater part of Phrygia and Mysia, we can surely speak of the failure of Pontic plans for Phrygia and the loss of the Mithridatids' influence in Galatia and the nearby states (Polyb. XXI. 41. 2). These reversals were patently the result of the Pontic support for Antiochus III. 2