[Sekto Springs]

Probably because Pen Ward, J.G. Quintel, and Thurop Van Orman are part of a creative troupe that work on each other's shows, so they're often spoken of in a collective sense. There are some similarities in their art and writing style, which is probably why the shows share a mutual fanbase.

Otherwise they're all fundamentally different programs that appeal to different sensibilities. Flapjack was more gritty, bizarre, and darker than the other two. Adventure Time is silly, lighthearted, and sort of arbitrary. Regular Show has shades of both, but it's far more "chill" and the humor is more subtle I suppose.

To give you an idea, here's a short film by Pen Ward (Adventure Time) that is pretty representative of his style.



And here's one by J.G. Quintel (Regular Show).