Bilbao beat Hercules to boost European bid
Martinez rose unmarked to nod powerfully into the top corner in the fifth minute at the San Mames, Llorente finished off a flowing move with another header in the 61st and Muniain pounced on a defensive mistake to make it three in the 64th.
Alicante-based Hercules rarely threatened the home goal and their best effort came midway through the first half when a header from Paraguay striker Nelson Valdez was saved low down by Bilbao goalkeeper Gorka Iraizoz.
The Basque club have 32 points from 20 matches, five behind Espanyol in fifth and two ahead of seventh-placed Atletico Madrid, whose erratic form continued when they lost 1-0 at struggling Sporting Gijon on Sunday.
Hercules, the only team to beat champions Barcelona this season, are 12th on 22 points.