You should have an idea of your own abiliyu and how much is going in.
You should have a timetable /study planner to keep you on target.
We all have different levels of abiliy, so whether 3hrs a day is enough depends on how lever you are, how much work youve done so far and how much you have to do. How goods do the grades you need have to be? I would do more than 3hrs a day tbh, but if you feel in control and you are getting good marks, then you have it right.