Let q=pr, where p is an odd prime and r is a positive integer. Consider a ring FqST, where S=Fq+uFq+vFq+uvFq with u2=1,v2=1,uv=vu and T=Fq+uFq+vFq+wFq+uvFq+uwFq+vwFq+uvwFq with u2=1,v2=1,w2=1,uv=vu,uw=wu,vw=wv. In this paper, we examine the algebraic structure of constacyclic codes over the ring FqST of block length (alpha,beta,gamma). Further, a construction of quantum error-correcting codes (QECCs) from constacyclic codes over FqST is also given. Moreover, we derive a number of new QECCs.