Let R=k[t(n1),...t(ns)] = k[x(1),...x(s)]/P be a numerical semigroup ring and let P-(n) = (PRP)-R-n boolean AND R be the symbolic power of P and R-s(P) = circle plus(i >= 0)P((n))t(n) the symbolic Rees ring of P. It is hard to determine symbolic powers of P; there are even non-Noetherian symbolic Rees rings for 3-generated semigroups. We determine the primary decomposition of powers of P for some classes of 3-generated numerical semigroups.