The Erdrick Trilogy, also referred to as the Loto Trilogy or Roto Trilogy refers to the first three games in the main Dragon Quest series. These games share a similar setting and timeline. In particular, they all involve the kingdom of Alefgard and the hero Roto.
Dragon Quest III is chronologically the first game in the trilogy. It tells the story of Erdrick leaving his native homeland and coming to Alefgard to defeat Zoma. After defeating him, Erdrick finds himself trapped in Alefgard when the pit he had used to access it from his homeland is sealed shut.
Dragon Quest tells the story of a descendant of Erdrick stepping forward to defeat a new evil foe who has taken up residence in Charlock Castle. The new Hero follows in his fore-bearer's footsteps and must emulate many of his actions in order to defeat the Dragonlord. Upon his victory, the hero and Lady Lora leave Alefgard to explore and found new kingdoms in foreign regions.
Dragon Quest II begins with an attack on Moonbrooke which is one of many kingdoms founded by the hero of Dragon Quest. In response, 3 descendants of Erdrick, 1 from each of the kingdoms of Lorasia, Cannock, and Moonbrooke, unite to defeat Malroth. They travel together throughout Torland, which is expanded Alefgard region until confronting Hargon and Malroth.