As the eldest son of Sancho III he inherited the dynastic rights over the crown of Pamplona, becoming feudal overlord over two of his brothers: Ramiro, who was given lands that would serve as the basis for the Kingdom of Aragón; and Gonzalo, who received the counties of Sobrarbe and Ribagorza.

Likewise, he had some claim to suzerainty over his brother Ferdinand, who under their father had served as Count of Castile, nominally subject to the Kingdom of León but brought under the personal control of Sancho III.

García Sánchez inherited the crown of Pamplona after the death of his father Sancho III in 1035, bypassing the late king's eldest son Ramiro, who was illegitimate.

García Sánchez III took advantage of the weakened state of the numerous Islamic taifa kingdoms that arose after the dissolution of the Caliphate of Córdoba to push the southern border over their territory, taking the city of Calahorra in 1045.

[2] The relationship between the two brothers would however turn sour by the conflictive distribution of the lands of Castile between León and Pamplona, leading to the Battle of Atapuerca, where García Sánchez would perish.