How can I make a game with three qualities? And how do I know what quality is suitable for the user’s device?
For example, I want to have three qualities, low, medium, high quality
When the user logs in, he can play the game in the appropriate quality according to the device he has, and if he wants to, he can change the quality
Is it necessary for all assets to be in three qualities or can the system itself do this with one asset?
No that’s something you need to handle in your code. Detect either the platform running (Android vs iOS vs Desktop) or detect the frame rate and accordingly decide on the preset. You can also attempt to detect the device GPU which will give you more hints on its capabilities, check this user manual page, at the end: