I have been using my S21 ultra this morning... not excessively... but setting up things and browsing. It is down to 74%... since 7:30am. i think that is reasonable.
You need to remember that the phone is learning your habits, and in fact the battery should be more stable after a number of days of usage.
If you go to settings - battery and device care - battery, then you will see where it has predicted time left ...there will be a statement 'A full charge will last about: Learning Usage Patterns'.
This signifies that it is getting used to your battery usage habits to optimise accordingly. I think you should be a little patient and wait until at least it has had a few full cycles of usage and charging before making any assumptions.
You are in the wrong forum. Try one that is relevant to the J3. Probably select category Smartphones - other smartphones.