The idea is they would be organized such that one kingdom was kept in reserve as a place of safety while the other two pushed into Moslem (and other Christian) lands in a hammer and tong rhythm.
Turned out they didn't really need to keep a kingdom in reserve. Carvajal was discarded as a kingdom in the first generation after the initial landing.
BTW, took King San Cho Noe I (King Sancho I) just a few weeks to totally bowl over the locals in Galicia, et al. They simply didn't have the technology to fight anyone.
Carvajal lived on as an order in France to which a French Knight who'd fought the Moors would be appointed by the King of France. It's symbol is a large red bull with big horns (or so I've had 'splained to me).
Actually, the Kingdom of Asturias far preceded both the Kingdom of Leon and the Kingdom of Castile.
The Kingdom of Asturias was established in 718 AD and had it's capital in Oviedo.
As the conquest moved south, the city of Leon, originally established by the Roman VII Legion (Legio Septima Gemina) was conquered in 742 AD. Over time, the old Roman name of "Legio Septima" had been corrupted to "Leon". Thus, the old Roman name for "Legion" morphed into "Lion".
As time passed, Alfonso III, King of León, divided his Kingdom into Asturias, Galicia and Leon between three sons, Ordoño, Fruela and Garcia upon his death in 910 AD. Thus began the "Kingdom of Leon" as a separate kingdom when it was inherited by García I of León. In the end, through the deathof Garcia and politics, Ordoño ended up ruling the united Kingdom of Leon, Galica and Asturias with the capital now in Leon.
Castile was originally newly reconquered territory on the eastern frontier of Leon and was just a county of Leon ruled by Counts of Castile that were, legally, vassals of the King of Leon.
With the passage of time, the Counts of Castile became more and more powerful until they were, in effect, independent although they remained, legally, vassals of the Kingdom of Leon. The rulers of Castile remained as "Counts of Castile" until Ferdinand I of León and Castile, Count of Castile, through civil war and marriage, gained control of both Castile and Leon and established the Kingdom of Castile in 1035 AD.
Sancho I of León (the Fat), was the son of King Ramiro II of León who assumed the throne of Leon in 956 AD and died in 966 AD. His reign was rather inglorious, being deposed once by the nobility led by the restless Count of Castile Fernán González.
The tale of knights from Cornwall might make a nice English legend but it is, in the end, just a legend with no basis in historical reality.