Chapter 3 has nothing much to do. A very small map with no optional quests (except elite and epic bosses) and does not develop on the overarching storyline - basically what galeoth was doing in chapter 3 is just exploring icy mountains. Which is unlike chapter 1 where we see the death of phodom's family that causes phodom to take revenge, it is also unlike chapter 6 where phodom kills xada to take revenge and the three heroes finish off the dragon who disguised as a demon to control kar'gath.
Next on priority is chapter 2 then 4. While they have interesting optional quests, however there is no development of the storyline. Chapter 2 is just about Fradz travelling and crossing a bridge. Chapter 4 is about Phodom passing through the Great Marsh.