I generally use this later one to see how is the latest strategies that the professional players are implementing later. Also on this site, you will see some big age of empires 3 streamers on the left side of the panel. They are good references.
Also, I guess Youtube is a good font. There are some great player that have youtube channels, some of then including:
Aussie Drongo was a professional player around 2008, and came back to the game recently. He is a high level Age of Empires 3 player that is probably and have the biggest youtube channel with content related to Age of Empires 3. He also do streams on Twitch (And is live right now).
SamuraiRevolution was the biggest AoE3 youtuber until some years ago. He stopped producing AoE3 related content for a while when the DE came out, but he is back at AoE3 again. His videos are a little bit old, and the majority is towards the Asian Dynasties version of the game, but are still worthy to check out.
Haitch is a professional AoE3 player, and have been between the 10 top players in the world from a long time now. He recently opened a new Youtube channel, and is doing videos contantly recently.
I guess those are the bigger and most important fonts out there for now. There are more places, but I don’t want to seen to much overwhelming.
And there is no up-to-date sites that gives stats from civilizations that I know about it, unfortunately.
I hope that this helped. Let me know if you need some more referencing.
Cavalry beats Skirmishers (includes archers) and Artillery
Skirmishers beat Heavy Infantry (musketeers, swordsman) and Dragoons (cavalry that fire projectiles)
Dragoons and Heavy Infantry beats Cavalry
Artillery wipes out all infantry
Jungle bowmen would be wreckt by cavalry then.
The only tricky part is memorized the Aztecs since they don’t have horses but they have units that act as cavalry and dragoons for all intents and purposes: