Thank you so much! We're touched to be the first (hopefully of many!) VN devs that get your support.
OBSCURA is a pay-once game. When you buy the game in early access, you're also buying access to all the future updates, which in this case is Chapter Three for all the characters. Those updates are automatically pushed to the game on Steam (and if you connect your Steam and Itch accounts in your settings then you can get the Steam key that's bundled with your purchase), and I believe also the Itchio app. If you're using the desktop site, you'll need to manually download updates when they happen, either by accessing the download page via the link at the top of this page or through your Itch library. But either way, you will not need to pay a second time to get those updates!