Storyline 360 is one of the tools you get with Articulate 360, which is indeed an annual subscription.
However, it's important to note that with your subscription you get more than just Storyline 360, you also get a huge content library which includes TONS of characters and literally millions of stock images, videos, icon sets and illustrations, as well as a Review app that lets you quickly share your courses for feedback with anyone. Plus there's lots more you get with an Articulate 360 subscription including Live training sessions every week with Articulate experts like myself, video recording & editing tools, and more. Also, with Articulate 360 you get new features and updates immediately as they're released, for example, the number variable which is already available in Storyline 360 but not in Storyline 3.
And yes, Storyline 3 is a one-time license. :) But just keep in mind that then you don't get all the other extras I mentioned above.
Hope that helps!