N.
I have a problem with anyone labeling a toddler a brat. To me, only someone fully aware of their actions can be "bratty". I don't think a 3 year old has the emotional awareness to know that her actions truly affect others. It's all about "me" at that age and that should be expected. They learn over time that this is not the case, but all children act "bratty" at that age at one time or another, but that does not mean they are a brat. Now, I know there are different levels of misbehaving and this person probably thought your daughter was misbehaving more than she should for someone her age, but it's all relative.
Did you ever see the episode of Everybody Loves Raymond where the kids' new friend "Spencer" was causing Ray so much headache. He was supposed to be this gifted child and his parents let him say and do almost anything while excusing it as just being "creative". Now that kid was a brat! Of course, he knew he was a brat and did it because he could.
I too think it's good for us to try to look at our kids from a different perspective on occasion. It's funny how some parents are hyper-critical of their kids and think that everyone is bothered by their behavior if it's even just a little disruptive while others think their kids are perfect and no matter what they are doing, everyone around them should just think it's as cute as they do. Again, all relative.
I think I'm realistic about my kids' intellectual abilities at this point. I know my 8 year old should NOT throw a temper tantrum in public because she wants a toy and doesn't get it, but I know that my 3 year old might and there's nothing I can do about it except try to distract him from the situation. I can try to explain it to him, but there's no guarantee at that age he will understand.
I say shame on that lady for saying that about your 3 year old daughter, especially where your FIL could hear it. She sounds like the brat to me.