  We propose to generate macroscopic superposition states of a large number of
atoms in the ground state of a three-mode spinor Bose-Einstein condensate. The
ground state is protected by a finite energy gap, is immune to phase noise, and
the measurement of the number of particles in one mode heralds the coherent
preparation of the two other modes simultaneously empty and filled. Highly
entangled macroscopic superposition states are generated with large probability
also taking into account quasi-adiabatic preparation of the ground state. All
the key ingredients necessary to realize our proposal are experimentally
available.
