It sounds like she could possibly have Obsessive Compulsive Disorder. Can she tell you why she repeats herself? Are her answers something like "because something bad is going to happen if I don't". Does she do any other habits repeatedly?
If your brother is autistic, then you probably already know that the spectrum is a very broad one. I work with autistic children in the classroom and I will tell you that a lot of them are highly intelligent while others are much lower functioning and can not add 2 + 2; but then, Einstein could not tie his shoes either, right? If your daughter has a pediatrician, they may be able to have her tested, if not, then before Kindergarten, the School District can do it; usually free of charge. There is also a possibility that she is a savant and a trained professional will know when she is tested if this is the case or not. Some kids may seem to have OCD which is also a symptom of autism, but simply repeating what you say is not necessarily OCD. She is only three-years-old, so she could also just be going through a unique phase. I wish you the best!
I know this is an old topic, but I wanted to post for others who may refer to this. I have a (now adult) cousin who does this. When he was younger he had trouble hearing and started repeating, in a hushed whisper, everything he said and what he heard. Its his way of making sure he gave the appropriate response to what he thought he heard. And also making sure he made sense to what was said. Sometimes its just a comprehension technique; sometimes they out grow it, and sometimes they don't. It doesn't affect anything, and could actually be beneficial to a parent or guardian. They repeat and you can correct if they misheard or you can clarify if needed based on reactions. My cousin never notices he does it, but kind of zones for a second as if reading it written in his brain, and then continues as if nothing happened. Dead useful when he was younger; their facial reactions to parts of the statement the second time around can indicate confusion or any other sentiment that we as people are trained to hide. A raised brow, brows drawing together (this can also happen when deep in concentration repeating and trying to make sure they got it), eyes squinting...all of it.
I wouldn't make a big deal out of it. It can be outgrown or used as a tool for better parenting. Perhaps if she is repeating in her normal voice, explain that it's okay that she does it, but she needs to keep it quietly to herself, either in whispers or in her head. I've never seen it be an issue, and my cousin is in the Army now, with a good paying job. He was never held back by or limited by his necessity to repeat.