Sancho III (c. 1134 – 31 August 1158), called the Desired ( el Deseado ), [a] was King of Castile and Toledo for one year, from 1157 to 1158. He was the son of Alfonso VII of León and Castile and his wife Berengaria of Barcelona, and was succeeded by his son Alfonso VIII.
Sancho III, (born c. 1134—died Aug. 31, 1158, Toledo, Castile), king of Castile from 1157 to 1158, the elder son of the Spanish emperor Alfonso VII. His father’s will partitioned the realm between his two sons, Sancho III receiving Castile and Ferdinand II receiving Leon.
Sancho III of Castile (1134 – August 31, 1158), called el Deseado (the Desired) due to his birth, as first child of his parents, not until eight years after their marriage, was King of Castile and Toledo for one year, from 1157 to 1158. He was the eldest son of King Alfonso VII of Castile and Berenguela of Barcelona.
Sancho Garcés III (c. 992–996 – 18 October 1035), also known as Sancho the Great (Spanish: Sancho el Mayor, Basque: Antso Gartzez Nagusia), was the King of Pamplona from 1004 until his death in 1035. He also ruled the County of Aragon and by marriage the counties of Castile, Álava and Monzón.
Sancho III (c. 1134 – 31 August 1158), called the Desired ( el Deseado ), was King of Castile and Toledo for one year, from 1157 to 1158. He was the son of Alfonso VII of León and Castile and his wife Berengaria of Barcelona, and was succeeded by his son Alfonso VIII.
Sancho III became King of Castile and Ferdinand II, King of León. The rivalry between both kingdoms continued until 1230 when Ferdinand III of Castile received the Kingdom of León from his father Alfonso IX , having previously received the Kingdom of Castile from his mother Berenguela of Castile in 1217. 
Death A Zamoran noble, Vellido Adolfo (also known as Bellido Dolfos), entered Sancho's camp pretending to be a deserter, assassinated him.   Vellido was chased back to Zamora by El Cid, but escaped into the town through a gateway since called Portillo del Traidor ("Gateway of the Traitor").
Died 31 August 1158 (aged 23–24) Toledo Burial Cathedral of Toledo Relationships Consort Blanche of Navarre
Sancho III of Castile is the 3,279th most popular politician (up from 3,321st in 2019), the 279th most popular biography from Spain (up from 286th in 2019) and the 99th most popular Spanish Politician. Sancho III of Castile is most famous for his role in the Reconquista. He was the son of Alfonso VIII of Castile and Eleanor of England.
Blanca married Sancho III of Castile, regent of Castile (subject to his father Alfonso VII) on February 4, 1151 in Carrión de los Condes, Palencia, after travelling from Calahorra, Logroño, in January.  The marriage was arranged to insure closer ties between León-Castile and Navarre.