Dd thought she would get 4 or 5 and didn’t and is so upset. What’ the best thing I can say to her? I know you studied so hard and felt you aced it but its curved and a 3 is still a great score?
I would suggest not talking about it at all. It’s done. Just move on.
In a week or so, she likely won’t be upset about this at all anymore.
If it was her 1st AP test- I would remind her that 50% of students don’t score (3+) on their 1st test- so she is already ahead of the pack
I wouldn’t even worry about it - DD crashed and burned on all three of her tests (even AP Government). Based on the college visits we had last week, many of the colleges she’s looking at would only allow her to use her AP score to credit out of electives and only certain AP classes were considered. The public universities she’s looking at will let her credit out, but they also warned that if she comes in as a sophomore based on AP/dual enrollment credits, she would be ineligible for many merit aid scholarships (and U of Nebraska warned she wouldn’t be eligible for the legacy scholarship she should get automatically as I’m an alumni.)
On the flip side, we haven’t had a single college yet tell her they wouldn’t let her credit out of a class for the dual enrollment history class she aced. 
Colleges care more about students taking a challenging curriculum than the exact score they got on the tests, though since you are asked to self-report scores it’s much nicer to get high scores. I think colleges know that some teachers prepare students better than others. A 3 is a good enough score at some colleges, but not any of the highly selective ones. A handful of colleges don’t give credit for APs at all, though they are happy to see you’ve challenged yourself.
You can tell her how proud you are of all her hard work. If she earned a course grade that she is proud of and that seems representative of her hard work and learning, you can remind her of that and say that colleges will look at the grade she earned in the class as indication of her ability and hard work. You can tell her that colleges know that the test is a one shot deal and know that everyone has off days. That is a big reason why you don’t have to report the score. Also if she has a good relation with that teacher and plans to have that teacher write her rec letter, the teacher can attest to her learning and achievement in the class. Mainly I would praise her hard work and learning in the course and mean it. Passing an AP exam is awesome and many students this week wish they had her 3.