Yup sounds like you got it figured out
All of our active development is in the 'develop' branch. That is currently on 3.2.0-SNAPSHOT. We haven't been very active in that branch specifically just because we don't have a lot of new features yet; we've been focusing on bug fixing.
Our bug fixing branches that are active are currently:
BroadleafCommerce-3.0.x (currently at 3.0.10-SNAPSHOT)
BroadleafCommerce-3.1.x (currently at 3.1.1-SNAPSHOT)
Those will end up getting released as 3.0.10-GA and 3.1.1-GA, respectively. Since 3.1.1-GA will be the latest version of Broadleaf, that will then become master. But you're right, we tag all of our releases like:
broadleaf-3.1.0-GA
broadleaf-3.0.8-GA
broadleaf-3.0.9-GA
...
You are correctly using the latest GA version of Broadleaf, 3.1.0-GA.
By the way, you shouldn't need to have Broadleaf checked out locally just to get to the sources. IntelliJ will download the sources automatically since we distribute them out to Maven central as well. You can open Broadleaf classes with cmd + shift + o (open file) and make sure you check the box 'include non-project files'. In Eclipse, this is called 'Open Type' (cmd + shift + t).