While play Caledonia, I have found that chapter 3 Main quest is to Subjugate 2 Factions or take their last settlements.
Bonus Objectives are to get trade with Gallic Rome, and Sack/raze a Germanic city (or something like that)
However the bonus objectives are fairly impossible to get.
This is because in order to complete Chapter/Quest 2, you need to control all of Britain and ireland.
To do this you need to defeat Britannia and the faction based in Ireland. Doing this triggers the quests for Chapter 3, however it already has the main quest ticked/completed, as you have defeated the other two factions to accomplish this, and ending your turn throws you straight into Chapter 4.
I have tested this with forcing the Irish to confederate, and completing chapter two still defaults to completing chapter 3 aswell. As I am assuming confederation is seen as a form of defeating a faction.
The only way this seems possible, is for you to hope Britannia and The irish faction go to war, Britannia wins and then you take it from them. Other than that Chapter 3 is impossible to complete with the side quests. Unless you somehow manage to plan ahead by raiding a german town, or manage to get trade with gallic rome while at war with britannia, theyre allies