Alfonso III was born around 1262 and was the
eldest son of Peter III.
Prior to the death of his father he was mainly concerned with
affairs on the mainland, 
while his younger brother James was
assigned to rule Sicily. Following his fathers death in 1285, he was
crowned king at Huesca on April 9, 1286. He proceeded immediately to
capture the last Muslim stronghold in the Balearic Islands on
Menorca and then attacked his uncle on Mallorca.
He also became involved in a war with Castile. As part of this conflict, he would release the infantes de la Cerda, who were claimants to the throne of Castile, in an attempt to undermine Sancho IV. If this was not enough, he was also embroiled with the unions of Aragon and Valencia, and was forced to make broad political concessions to them.
Initially, Alfonso intended to hold on to Sicily, and it was under Alfonso that the Office of the Admiral reached its zenith. However the situation demanded he disentangle himself from the island. On October 29, 1288 he released Charles II from captivity as part of the agreement of Canfranc. However, Charles went back on his word and was crowned King of Sicily in Naples. Alfonso responded by besieging Gaeta.
On February 12, 1291 he would agree to the Treaty of Tarascon in an attempt to end the war. He would also sign a nuptial agreement to marry Eleanor, daughter of Edward I. Neither the treaty nor the marriage would take place, for he died on June 18, 1291. Having no heirs, his younger brother James would become king.
