Some link suggestions:
With this guide I learned the basics of Modding for AOE3:
On these pages are collections of links to guides that are outdated but may be of some use:
Modding - Age of Empires III Heaven (heavengames.com)
The Tutorial Database - Age of Empires III Heaven Forum (heavengames.com)
In this topic (in the comments) I explain a few things about the AOE3 mod that might be useful to someone:
Campaign Civilizations - Age of Empires III: DE / III - Modding - Age of Empires Forum
(I don’t have time right now to add this myself, so I’m leaving these links here in case someone wants to include them on the wiki somehow… but if I have some time to answer questions about it)