Grandson of Demetrius Poliorcetes & cousin of Demetrius II,  who -- after the latter died in battle -- rescued Macedonia & restored Antigonid control of Greece. Modern encyclopedic articles sometimes mistakenly identify him as either the "half-brother" or "nephew" of Demetrius II. Given the complex family relations of Macedonian dynasties & the tendency of Macedonian rulers to give children of different wives the same name, such confusion is understandable. Doson's father, however, was actually Demetrius the Fair [briefly king of Cyrene], the son of Demetrius Poliorcetes & his third wife, Ptolemaïs [daughter of Ptolemy I & sister of Ptolemy II]. It was Doson's father [Demetrius the Fair] who was half-brother of the father of Demetrius II [Antigonus Gonatas], who was Poliorcetes' son by his first wife, Phila, Antipater's daughter & Cassander's sister [see chart below]. Beyond this fact there is only circumstantial inference from the few surviving anecdotal reports to clarify Gonatas' background.

As Demetrius Poliorcetes' namesake, Demetrius the Fair was not only strikingly handsome but ambitious to the point of wrecklessness.  As the grandson of Ptolemy I Soter, he had been summoned from Macedonia to marry Berenice, the heir to the vacant throne of Cyrene [modern Libya]. But rather than content himself with his young bride, Demetrius openly became lover of her powerful mother, Apama. So the jealous bride took her revenge by having him assassinated.

It is highly improbable that Antigonus Doson was himself born of this ill-fated Cyrenian venture. Given the lurid circumstances of his father's death & the lack of any hint that he himself was in line for any throne or had ever been in north Africa, it is more likely that Doson was Demetrius' son by a prior, less politically advantageous marriage & was left in Macedonia when his father set off for Cyrene. 

Even the meaning of Antigonus' by-name is obscure & uncertain. Plutarch's suggestion that it indicated that he did not live up to his promises can hardly be correct since, once fate placed him in a position of authority he used it more wisely & effectively than any of his Antigonid predecessors. 

Nor is there any evidence that he was a power hungry usurper. Rather, the sources are clear that, when Demetrius II died in battle (229 BCE), the Macedonian nobility named Doson guardian of the nine year old heir apparent [Philip V], presumably since he was the child's sole surviving adult male relative.  Only after Doson demonstrated his leadership abilities by succeeding (where his cousin Demetrius had failed) in expelling the Dardanian invaders, was he persuaded by other Macedonian leaders to claim the throne for himself by marrying the widowed queen (227 BCE).  Apparently, both the Macedonian army & nobility thought the political situation too volatile to wait for Philip V to mature enough to assume command.  Times of political crisis & uncertainty always call for firm direction from experienced leadership.  And in this crisis Antigonus Doson was Macedonia's unanimous choice.  Unlike his Antigonid ancestors, he had no viable rivals to challenge his right to rule. Yet, even as king he apparently envisioned himself as caretaker for his cousin's son, Philip V. For he was never accused of trying to make his own sons  heirs to his throne.

 As king, Antigonus III proved to be as much a master of tactical diplomacy as of military strategy. In less than a decade of rule he not only secured his nation's borders, he also reestablished it as the dominant power in the region.  Unlike previous Macedonian rulers who attempted direct dominion over their fiercely independent neighbors to the West & South, he formed alliances with Epirus & the Achaean league.  When Sparta, the historic rival of the latter, attempted to establish hegemony over the whole Peloponnese, Aratus of Sicyon -- long the leader of Greek opposition to Macedonian domination -- invited Antigonus to intervene (226 BCE). Establishing his base on the heights above Corinth, Antigonus reconstituted a broad-based Hellenic league (224 BCE) under his leadership before launching his attack on Sparta.  Outmatched by the larger, better equipped Macedonian army, the Spartan forces were so overwhelmed in the battle of Sellasia (222 BCE) that their king [Cleomenes] had to seek refuge in Egypt.  

Greece was finally at peace. But Antigonus had no time to show how he would exercise leadership over a pan-Hellenic confederacy. For, while his forces were campaigning in the southern Peloponnese, Illyrians [from modern Croatia] invaded Macedonia from the north. Antigonus had to rush north to repel this new threat. But though Macedonian forces were again victorious on the battlefield, their commander died of a ruptured artery as he was shouting orders to his troops.  

References: Justin, Epitome 28.3-4.
                  Plutarch, Aemilius Paulus 8;
                  _____, Cleomenes 16, 20-21, 23, 26-30, 34;
                  _____, Aratus.

Other resources on line:

• Justin's Epitome of Pompey's Philippic History - book 28 recounts the end of Demetrius II & the career of Antigonus Doson [Corpus Scriptorum Latinorum].

• Aemilius Lucius Paulus, Cleomenes & Aratus - Dryden translation of Plutarch's Lives annotated by David Trumbull [Internet Classics Archive].

• Antigonus III Doson - entry in Wikipedia's web.

Phthia
of Epirus 

Note

The table above is a pruned family tree, representing the relationships of the ruling members of the Antigonid dynasty [gold] & their spouses [=]. For a full genealogical table of the offspring of the marital alliances of all ancient Macedonian rulers see Kelley L. Ross'  Hellenistic Monarchs down to the Roman Empire in his comprehensive Historical Background to Greek Philosophy [posted on the Proceedings the Friesian School website].