I personally think that 3 hours a day isn't enough. I've been doing around 5-6 hours a day and i am perfectly sane lol. But i do think there is a limit when you have to say 'enough is enough', as Mao explained. I've always wanted to do 8 hours a day, but by the time i get to 5-6 i just get too tired. I think it would be easier if you had 2 or more subjects to study for because it mixes it up a bit.
For example, this week i am studying Media and History and will probably do around 6 hours a day. I'm fairly confident for History but not so much for Media, and will probably spend all that time revisiting the necessities of the course and completing copious amounts of practice questions. IMO, if you have a clearly defined goal then study as much as you think you need to. I am just waiting for the day after my Media exam which is only 8 days away, so i feel as though i should give it everything i've got until then.
However, if you are in year 11, don't go too hard because you've got a big year ahead.
What a great rant.