It's like BSD but if the goofy/serious ratio was set to 95/5 (rather than somewhere closer to 50/50), and there are aliens.
Has similar characters (same archetypes/roles) and setup (ie. gang v. gang v. gang/baddie).
The largest difference between the two is what the series focuses on. Main series BSD focuses more on story telling and character building while B3 focuses more on world building and the day-to-day shenanigans the protagonist team face.