Kingdom Hearts 3 is a long game that encompasses 9 different Disney and Pixar worlds which you can visit, many of which can last several hours at a time and involve lengthy cutscenes. Therefore it is important that you know how to save the game correctly as there is so much potential for lost save game data between those cutscenes.
Save Points
Using the Save Points that appear in most of the larger areas you enter is the most obvious way to save your progress in the game. These Save Points tend to be placed before and after big boss fights and are indicated by green portals which you must interact with in by walking over them in with Sora and press the button (Triangle on the PlayStation 4 and Y on the Xbox One) that shows up in the bottom left hand corner of the screen in order to save. Once you press the save button you will open up a menu screen that will prompt you with choosing a specific save slot. Note: Even if you do not intend to save the game you will find that walking over these green portals will instantly replenish your guages.
Autosave
Unlike other games in the series Kingdom Hearts 3 has an autosave feature where the game will autosave during gameplay sections. The length of time between saves varies and is never longer than a few minutes.