As we continue our adventure of the webpage toolbar, the next link we come to says "Season Three: Archive". This is like a log for the webcomic Season Three. It lists all the pages of the comic and how to get to them 'n' stuff. If you take a look there (be sure to ctrl+click lest you lose this handy tutorial), you'll see that there are two links: Part One and Part Two.
Season Three is separated into a bunch of little sections called "Acts". These are like the acts of a play. In fact, that's pretty much where the name comes from.
Part One of Season Three is the Prologue through Act 4. Part Two begins with Act 5 and hasn't been finished. I'd hint at how many acts are planned, since I happen to know that information, but I'm not sure how much the author has revealed.