I bought it and downloaded all of the 3 files and now I can't get the game to open. Please help
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You should be able to download the Installer from right here on the itch.io page (see the "Download" button above the comments section).
Run that installer to install the game, if you haven't already. The installed game should be reachable from the start menu after that.
One of the extra 3 files you get for buying the game is the key you use to unlock the full version of the game (from the demo version you installed).