The Catalans, who are aiming to win the trophy for a joint-record fourth year running and 30th time overall, picked a heavily rotated side without Lionel Messi, Luis Suarez or captain Andres Iniesta, but made light work of their third-tier opponents.
Paco Alcacer scored the opening goal in the 44th minute, steering a header home after Gerard Deulofeu carved out a cross.
Deulofeu, Barcelona's most lively player, netted the second just six minutes after the break with a superb individual dribble and tidy finish.
Highly rated Barcelona B striker Jose Arnaiz slammed home the third from the edge of the box in the 56th minute on his debut, to put the game beyond the hosts.
Elsewhere, there was a surprise in Soria as second division Numancia stunned Malaga 2-1 thanks to two goals in stoppage time, meaning the sinking Andalusians are yet to win this season.
Sevilla thrashed Cartagena 3-0 away from home, Valencia earned a 2-0 win on their travels at Real Zaragoza, Getafe went down 1-0 at home against Alaves and Cadiz were defeated 2-1 by visitors Real Betis.
Atletico Madrid get their King's Cup campaign underway on Wednesday when they head to Elche, while Real Madrid don't have far to travel to face Fuenlabrada on Thursday.
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