(be mindful if you're new to AC3 that the proper game starts about Sequence 6 or 7 for this guide)

So money grinding in AC3 is very different than in the Ezio trilogy, Connor is not a dirty capitalist like Ezio who monopolizes entire cities, makes shopkeepers & workers funnel his banking while he himself may run around Rome physically stealing money out of the pockets of the proletariat.
Instead, Connor is a huge cog in the manufacturing & fur trade bits during the Trans-Atlantic Slave Trade.

1) Beaver pelts
...are the easiest to get to & second highest valued pelt trading good. There is a reason beavers were hunted to near extinction: they are accessible on the right bank of the river south of the Homestead main building, about 7 or so.

2) Bear pelts
... is what you ditch beavers for as soon as you can. Generally speaking they give roughly the value of 1.3 beavers at minimum. They require you to either go through a sequence of QTEs or for you to awkwardly jump at them just in time to 1 hit air assassinate from ground level.
(the QTE are a randomized combination of E then Space, E then LMB, E then Q)

Bears can be found at what I like to call The Ultimate Hunting Spot, you could also call it Bear-Prime.

https://gtm.steamproxy.vip/ugc/1693905492551020077/3B92CB6F0FB1CE18E32E5DA79DB192363BC9773B/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Red: The Ultimate Hunting Spot
Green: Quickest way to get there from the nearest fast travel

Picture Description in case Steam breaks for you: It's a pond north-west of the harbour fast travel in the Frontiers, topside of a waterfall with a ruined log cabin.

This location in the Frontiers contains anywhere from 4 to at best maybe 8 bears if you're really good at running from one spawn point to the next killing them. If you run out of them by any chance or you want to get really cozy, use tracking evidences: They tend to spawn in 1 new animal per evidence if you hunted existing bears. If you sniffle one evidence, the next ones spawn additional animals.

3) Send carts and later on ships to shill pelts. You can casually do whatever you want in between sending them, but never sell pelts yourself in stores (it's 160$ per furry versus ~750$ if you send carts for example...).

Pretty much everything else is up for you to play around with, this is just a simplified guide in case anybody needs that.