Youth and succession
Peter was the eldest son of James I of Aragon and his second wife Violant of Hungary. Among (opportunistic) betrothals of his youth, he was betrothed to Eudoxia, the youngest daughter of Emperor Theodore II Laskaris of Nicaea, in or before 1260. This contract was dissolved, however, after Eudoxia's brother lost the imperial throne in 1261, and Eudoxia was instead married to the Count of Tenda. On 13 June 1262, Peter married Constance, daughter and heir of Manfred of Sicily. During his youth and early adulthood, Peter gained a great deal of military experience in his father's wars of the Reconquista against the Moors.
On James I's death in 1276, the lands of the Crown of Aragon were divided amongst his two sons. The Kingdom of Aragon, the Kingdom of Valencia and the Catalan counties went to Peter III as being the eldest son; while the Kingdom of Majorca (actual Balearic Islands) and the Catalan counties beyond the Pyrenees of Rousillon-Vallespir, Conflent and Capcir and the lordship of Montpellier (all his territories in the Languedoc), went to the second son, who became James II of Majorca.
Peter the Great and Constance of Sicily, Queen of Aragon were crowned in Zaragoza (capital of the Crown of Aragon) in November 1276 by the archbishop of Tarragona.