The third chapter is where the second half of crusade management is introduced and about 70% or so of the world map opens up.
Basically it’s a much simplified version of Kingmaker‘s kingdom management. You get advisory councils made up of various party members which let you make decisions to raise different sorts of stats and/or recruit different units. Aside from overall army morale, you don’t have to ever worry about a stat decreasing here or encountering a negative event that will destroy your city if ignored.
As for buildings, they all pretty much now either boost your army or resource gathering potential. So (after a Teleporter) you’ll want to build anything that increases resources or reduces costs first, followed by anything that increases recruitment for your preferred troops, and lastly anything that boosts those troops’ effectiveness.
Exploration-wise it’s pretty much identical though, meaning that visiting areas in the ‘wrong’ order is quite possible and can have you running into challenges you’re not yet prepared for. You’ll want to focus on your mythic quests first (which may or may not be immediately available depending on path), followed by the Greengates and Molten Scar-related questlines (along with any companion quests that trigger). After those you can take on the dragon, Wintersun, and the Ivory Sanctum. Blackwater should be saved for last because the enemies there are quite a bit more powerful than anything else in the chapter.
If you want to visit every area efficiently (which is far from necessary since most sidequests only show up after a certain amount of time has passed), then this order will clear the map in 2-3 weeks depending on resting habits:
Ruins of Ashbury Hamlet, Temple of the Good Hunt, Ashen Grotto
Shrine of Sacrilege, Greengates, Molten Scar, Pitfall (Reagents)
Scrubland by a Bend in the River, Broken Cart (Reagents), Grimwood
Sacred Lands, Bloody Trail, Artisan’s Tower, Wintersun
Ravaged Longhouse, Terendelev’s Lair, Heart of Mystery
Burned-Down Shack (Reagents), Befouled Barrows (Storybook)
Knapsack on a Tree (Reagents), Core of the Riddle
Remains of a Disgusting Feast (Reagents), Crimson Dust, Ivory Sanctum
Makeshift Lab (Reagents), Legacy of the Ancients
Petrified Traveler (Reagents), Blackwater, Bones in a Pool of Mud (Reagents)