  The Lagrangian Sp(3) BRST symmetry for irreducible gauge theories is
constructed in the framework of homological perturbation theory. The canonical
generator of this extended symmetry is shown to exist. A gauge-fixing procedure
specific to the standard antibracket-antifield formalism, that leads to an
effective action, which is invariant under all the three differentials of the
Sp(3) algebra, is given.
