He was born in Valencia in 1265. The bigger son of Pedro III of Aragon and his wife Constanza from Sicily, daughter of Manfredo I of Sicily. He was nicknamed "The Liberal." To his father's death he inherited the territories of the Crown of Aragon, while his brother Jaime II of Aragon inherited the Kingdom of Sicily.
     In 1282 he governed the kingdoms of his father when this one went to the conquest of Sicily, later to take charge of the government of the insular kingdom. In 1285 he took part in the defense of the Catalan counties in opposite to the attack of Felipe III of France.
      Between 1285 and 1286 he conquered, for order of his father, the islands of Ibiza and Majorca of his uncle Jaime II of Majorca, staying the kingdom of Majorca as tributary from Aragon. In fact when Pedro III died in Villafranca del Penedés, his son Alfonso was in Majorca to the control an expedition to punish his uncle Jaime II of Majorca for the help that had given the French monarch Felipe III in his attempt of invading Catalonia in June - October, 1285. Finished the expedition, Alfonso III returned to Alicante.
     After the arrival to Alicante after the conquest of the Kingdom of Majorca, he spoke to the city of Valencia, where was sworn king on February 2, 1286. Later king of Aragon was crowned in the Cathedral of Saragossa on April 9, 1286 by the bishop of Huesca, before the absence of the archbishop of Tarragona, who had to crown the kings of Aragon from times of Pedro II. Although, in fact, no king of Aragon was crowned by this ecclesiastic charge.
     In turn, and already like king of Aragon, he prepared the expedition to take the island of Minorca to the almojarife (kinglet) Abû’Umar. This way, in the Spanish Parliament celebrated in Huesca on October 18, 1286, he summoned their citizens in Salou, from there, to leave to the conquest of the island.. Minorca had been traditionally a vassal of Aragon from times of Jaime I (1232), but the king of the island was accused of being allied by Tunisia -and indirectly with France- and of giving support to diverse North African places, moreover having turned into a pirates' refuge that were obstructing the commerce. On November 22 there went out the ships that did scale in Majorca, where remained till after Christmas. The forces - that are calculated took part in the expedition - were provided with approximately 20.000 men and more than 100 Catalan, Aragonese and Sicilian ships. They arrived at the Nankin port on January 5, 1287, but the Christian troops did not disembark up to 17 of the same month, day in which at present the "day of Minorca" is celebrated. Although the Moslem troops opposed resistance, were forced to be refolded to the castle of Sent Agáyz (Santa Águeda) and to request the rendition.
      On January 21 the Agreements of Sent Agáyz were signed, for that the inhabitants of the island would be slaves of the king of Aragon; and all their assets, except the clothes, taken over unless they were paying seven "doblas" (like a doubloon) and half in golden in the course of six months. Those who could not pay were sold like slaves on markets of the north of Africa, like Bugía or Tripoli. King Abû´Umar was allowed to leave the island towards Barbary with 200 familiars or close friends, moreover his library, the mortal remains of his father Said Ibn Hakam, fifty swords and trousseau for the trip. The island remained deserted, his grounds were distributed between the nobility and the repopulation ran in charge of Catalans. He remained in Citadel for 45 days, where he dictated the guidelines for the government of the island, and ordered to construct the Cathedral-Church on the ancient mosque, although its construction started a bout 1300 when Alfonso III had already died.
      The Mediterranean ambition of the monarch provoked a despondency in the Aragonese nobility that felt wrongly placed, for example, for being proclaimed king before swearing the Jurisdictions and that saw questioned his privileges before the victory of the monarchy. This way, the Spanish Parliament celebrated in June, 1286 in Saragossa and those of October of the same year, in Huesca, finished with countermeasures and military actions between the monarchy and the noblemen, who threatened to give the throne to Carlos de Valois, whom the Pope had named like sovereign of the kingdoms of Pedro III.
      To improve his relations with the nobility, in 1288 igranted the General Privilege of the Union, for which he was promising to summon annually, in Saragossa, Spanish Parliament that would be those who would designate the Council of the king, and not proceed against the Union without previous judgment to the Justice of the kingdom and of the permission of the Spanish Parliament. If the king was acting against the stipulated in this document, the noblemen could "desnaturarse" (to deny the obedience and to choose other sovereign without incurring infidelity note). This condition gave motive to Alfonso III to be said: " ... that ... there were in Aragon so many kings as richmen ".
     In the Spanish Parliament celebrated in Monsoon in 1289 and thanks to the support of the Valencian and Catalan classes, he managed to reinforce the power of the monarchy against the power of the noblemen.
      He carried out an antiCastilian politics continuing, the politics of its father Pedro III, due to the passivity that maintained Sancho IV of Castile opposite to the French attack of 1285; since the Castilian king did not give the engaged help. It favored Alfonso and Juan, infantes of the Cerda, in their pretensions to the Castilian throne, receiving them in their domains, and ending up crowning the bigger of these, Alfonso, as king of Castile and of León, in Jaca in September, 1288 and being guarantor of this in his dealings with the king of Granada.

     This support to the infantes of the Cerda ended in in several border fights in April - June, 1289, September, 1290 and February, 1291, after having agreed on a friendship agreement with the benimerines. In compensation, Alfonso de la Cerda committed to transfer the kingdom of Murcia to Alfonso III, but such a transfer was never effective. The above mentioned infantes were basing their pretensions on the illegality of the marriage of Sancho IV of Castile with Maria de Molina.
     He ordered the naval expeditions of Roger of Lauria, Bernat of Sarriá and Berenguer of Vilaregut.
      His biggest problems in the international scenario, were his conflict opposite to France, the Valois and the Papacy for the rights of his brother Jaime on Sicily, whom in the first instance supported despite the foreign pressures. The Papacy, which had excommunicated before the kings of Aragon, until the kingdom of Sicily was going on to hands of who he was thinking like a just, he donated the kingdom of Aragon to Carlos de Valois and imposed ecclesiastic censoring.
     He faced the French invasion of his territories on the part of Felipe III of France, whom he conquered in the battle of the hill of Panizas (1285).
     He received Pope's ambassadors and of the kings from France and England that requested the liberation of the prince of Salerno, future Carlos II of Anjou, call '' the lame'', to who his father had made a prisoner. Alfonso let in freedom to Carlos, October 29 1288 making him to give up the Kingdom of Sicily in favor of his Jaime brother and leaving like hostages in Barcelona, to his two sons, Luis and Roberto.
      Finally and after the impossibility for Carlos II of Anjou, of taking Sicily, he went over to the signature of the Treaty of Tarascón in 1291, in which the conditions imposed by the Papacy were revoked. With this agreement the excommunication was getting up to the king of Aragon and he was recognizing the rights of Aragon on Majorca, while Alfonso gave up the Sicilian throne, was promising to act so that Jaime would resign from the kingdom of Sicily in favor of Carlos II of Anjou and even to do the war to him in case he was refusing to transfer his throne, despite the opposite position of the Sicilian; and for paying again the tribute to the Church and to satisfy the back payments. Since Jaime did not resign from the throne of Sicily, there was approaching a new conflict, which did not take place for Alfonso's sudden death.
     He continued the politics of his father in Africa and East, trying to establish linkage points between the peninsula and commercial ports of the North African coast and of east. In 1286 he submitted to vassalage the sultan of Tremecén and tried the same with that of Tunisia, but without success. For this, he threatened to these territories with an alliance with the sultan of Morocco who also was aspiring to these territories.
     In 1290 he signed a commercial agreement with the Byzantine emperor Andrónico II, who was authorizing all the merchants of the territories dominated by Alfonso III, to trade with the empire, paying at the entry and exit of the products a valuation of three per cent.
     


     He married for powers infanta Leonor, daughter of Eduardo I of England and Leonor of Castile. The marriage was celebrated on August 15, 1282 in Barcelona, but it does not go so far as to be completed, since when it was preparing the betrothal he suffered a glandular infarction that brought to him the death in only three days. He died on June 18, 1291, being 27 years old, in the city of Barcelona, without leaving descent. He bequeathed the kingdoms of Aragon, Valencia and Majorca and the Catalan counties to his brother Jaime, king of Sicily, under the condition of which this would resign from this kingdom and would be transfered to his another brother Fadrique. His successor would be Pedro IV.
     In the Divine Comedy, Dante tells that he saw the spirit of Alfonso III sat down outside the doors of the Purgatory, with other monarches whom he accused them for the chaotic state of the Europe of the XIIIth century.
      Alfonso III were buried in the missing Convent of San Francisco of Barcelona, place of sepulchre of the Aragonese royal family's members. In 1835, San Francisco's Convent was demolished and the king's remains, together with those of most of the queens and infants there buried, were transferred to the Cathedral of Barcelona, including the remains of mother of Alfonso IIII, Constanza of Sicily, where nowadays are placed in two sepulchers, executed by the Catalan artist Frédéric Marès. In the sepulcher where the remains of Alfonso III rest they are also the spoils of Jaime I of Urgel, count of Urgel and son of Alfonso IV of Aragon, and the remains of infant Fadrique, dead in the childhood and son of Alfonso IV "el Benigno".