Quality not quantity.
If you are revising at 100% efficiency maybe 24 hours a week might be enough for now, but you will certainly have to increase that over time. It will be very difficult to achieve good grades in all three subjects if you are only devoting 8 hours per subject per week, which would only be an hour and 20 minutes per subject.
There are a lot of factors though, which do affect how many hours you need. What subjects you're doing, what grade you're aiming for, and such all are significant factors which determine how much you should be working.
For example, a friend of mine who achieved A*A*A*A* at A-Level did Maths, Further Maths, Economics and Chemistry; all of which are quite heavy subjects, and so he worked heavily every day devoting at least 6 hours a day for his academic work.
Another I know of who achieved A*A*A* did photography, graphics, and art, which are not considered as heavy as the other subject combination above; and he studied slightly more than you're already doing.
Also, note that the amount each person revises also varies. The guy above me stated he studied maximum 2-3 hours every day, which may not be the case for you. You might require more than that - try not to compare yourself with others imo. Maybe continue with this current number of hours and when you take mock assessments during the year and stuff you might realise whether you're working too little or enough.