Kingdom Hearts 3 is quite long and sometimes a demanding game. On many occasions, you'll fight bosses who are really powerful. Fortunately, you can save the game and, if you lose, replay the stage or load the game and go grind. On this page of the guide, you'll find a short summary of how to save the game in KH3.
To save the game, you need to reach a green, spinning circle, often placed in safe locations. Here you can save the game, restore health and replenish supplies at the nearby Moogle Shop. From time to time, it's worth saving the game on a different game slot in the case your save gets corrupted. Save whenever you can to avoid replaying longer fragments.
Additionally, Kingdom Hearts 3 is a linear game where you don't make any choices, so you don't have to worry about loading a game after choosing a "wrong" option.