Most people follow BigMadAndy's way of thinking, which is very reasonable. Let me be the devil's advocate here. In your mind, you're still with the job/interviews and cannot let go of the possibility of getting a response. Well, if not asking probably won't give you the job, why not try to get an answer to your question?
Although this might be different between countries, I would wait for a certain time (you've been waiting two weeks now, that's fine) and then find a way to contact them. The reason and medium for this contact can be different as well:
- If you have a telephone number, call them. You'll have a higher possibility of receiving an answer and, if you've been nice at the phone, your name rings again in their minds
- If you don't have a phone number and can only send an email, that's the second option. Often emails are not responded to (which I still find very inappropriate, especially after actual interviews), so don't expect an answer. But: nothing ventured, nothing gained.
- Asking for the third interview is probably a bad idea
- Asking for whether they made a decision is too. They either made up their minds already and didn't send you an email or haven't made a decision yet (and you might get onto someone's nerves as BigMadAndy mentioned).
- Asking when you could expect a response to your interview of two weeks ago, is a lot better. You're not asking for the job, you show that you still have an interest in it and you're kindly asking for when you can expect a response.
This last option can still lead to no answer, but at least you tried. Or it can return a 'you're not the one', which is still a response you would like to have. Or they respond they need some more time. That's the most hopeful option.
Again, this might be seen as a bold action in some countries (in the Netherlands, we're often seen as direct). Both seeking for an answer to your question and waiting until they come to you have risks, advantages, and drawbacks.