  A constructive scheme for determining pure states (clusters) at very low
temperature in the 3-spins glass model on a random lattice is provided, in full
agreement with Parisi's one step replica symmetry breaking (RSB) scheme. Proof
is based on the analysis of an exact decimation procedure. When the number c of
couplings per spin is smaller than some critical value c_d, all spins are
eliminated at the end of decimation (RS phase). In the range c_d<c<c_s, a
reduced Hamiltonian is left; each ground state (GS) of the latter is a "seed"
from which a cluster of GS of the original Hamiltonian can be reconstructed.
Above c_s, GS are frustrated with an energy per spin larger than -c. The number
of GS in each cluster, the number of clusters, the distances between GS are
calculated and correspond to RSB predictions.
