The thing to do here is to look at what is different about these three phrases, and then pop over to a dictionary like www.leo.org to see the most direct meanings of these different parts. That way, we can try to see how they might be used differently.
So, for Das ist wahr, we can look up wahr and see that it generally means "true." Thus, the core meaning of this phrase is "That is true."
The meaning of the verb stimmen that most fits this context is "to be right," so Das stimmt essentially means "That is right."
Finally, korrekt is pretty easy: it commonly translates to "correct." Thus, Das ist korrekt is "That is correct."
Now that we know the essential meanings of these phrases, we can see that, just like in English, there are going to be situations where Das ist wahr "That is true" sounds perfectly natural; also, there are probably going to be situations where saying Das ist korrekt "That is correct" might seem a bit robotic. Das stimmt "That is right" is likely the most flexible, since its core meaning is a little more general than the other two.
So when you have different phrases like these, check out what it is that makes them different and try to keep their core meanings in mind when you have to choose between them in a conversation. Also, just keep an eye (or ear) out for how native speakers use them, and you will start to get a feel for them over time.
I hope that this was helpful!