I feel like a practice schedule is a very personal thing, and there never can be a concrete answer to "how much" is "enough." As others have said before, it all depends on your goals and what you hope to accomplish. If you feel like you should be practicing more, then practice more, but only as much as you're physically and mentally capable of. Past a certain point and you do more harm than good. Any time over two hours should realistic be broken up into smaller chunks of time anyway.
That said, you say you practice three hours a day and seem pretty disciplined (I try to have a set goal "every time" I sit down to practice, but alas, I'm, well... human), so, I have to ask, what prompted you to make a thread about this? It sounds like you have your routine pretty well established. What is it about your progress/practicing that you're unsatisfied with? Perhaps if we know the concern, we can better address the remedy.