  A $Z_3$ orbifold compactification of $E_8\times E_8^\prime$ heterotic string
is considered toward a trinification $SU(3)^3$ with three light families. The
GUT scale VEV's of the $SU(2)_W\times U(1)_Y\times SU(3)_c$ singlet chiral
fields in two sets of the trinification spectrum allow an acceptable symmetry
breaking pattern toward MSSM. We show that a doublet-triplet splitting is
related to the absence of a $\Delta B$ nonzero operator.
