Check out this demo:
https://howtoconstructdemos.com/save-and-load-game-progress-to-a-disk-file-capx/
If you don't want to save to files, and only want to give players 3 save slots, it's also possible. Simply save to "Game1", "Game2", "Game3". But you may want to keep track of which slots exist and which not.
I see. Very helpful, thanks!