For the next three years, department stores such as Dillard's, Robinsons-May, Saks Fifth Avenue and Kohl's will continue to grow more slowly than other kinds of retailers. Discounters such as Costco, Target and Wal-Mart will see the overall highest same store sales growth. But dollar stores such as Dollar General, Dollar Tree and Family Dollar will see the highest gains in sales, according to company reports and JP Morgan estimates.