After you get your first house in Webwood and your second house in Adessa, your in for a treat for your third house. Following the main quest line will eventually take you across the water to Mel Senshir, where you face the most epic of battles. After this mission, you will have access to Klurikon a.k.a. the entire eastern side of the map. Head immediately for the Caeled Coast. It consists of two areas, go to the east side of the southern area and look for the fortress of Seawatch. Go inside, find the quest starter, and begin the quest called "Ghosts of Seawatch". You'll have to find a few people to bring back with you to finish the quest. When you do, you'll be rewarded with the most badass digs in the game. Complete with alchemy, sagecraft, blacksmith and even a merchant. Best house in the game by far!
If you do this right away, you'll have access to this house throughout the entire second half of the main quest.