This ad was published in January 1989 (nearly a year after DQIII and a year before DQIV.) It proves how popular this series was in Japan that all three earlier titles were still being advertised during a 2 year break between installments, and judging by the prices in this ad, were still being sold at full retail prices similar to new games.
Dragon Quest, Dragon Quest II, and Dragon Quest III are all RPGs developed by Chunsoft and published by Enix for the Famicom.
Dragon Quest released May 27, 1986
Dragon Quest II released January 26, 1987
Dragon Quest III released February 10, 1988