Hi E....I'm A.
Maybe my personal experience will help you with your situation....
I am the very proud mom of a 10 yr old 'special' angel girl (and 7 other children). She has Asperger's Syndrome, which is a mild form of Autism. Since day 1, we've told her that she is special, different...not disabled or that there is something 'wrong' with her.
Yes, children are naturally curious (so are adults...lol).
When my 'special' lil one was very small and we noticed that she wasn't doing all of the things that my other kids did at her age...we had her tested. From the moment she was diagnosed, we started teaching our older kids that she was going to be learning to do things a lil differently than they do things. We didn't make a big deal out of it. What helped alot was reminding them that we all do things differently. Things as small and simple as walking and talking..none of us walk or sound exactly the same. "Special' people are simply doing things different. Altho you may think that your 3 yr old is too young to understand, I can tell you from experience that she knows and understands more than you realize. The word disability simply means that one is not able correct? Well, I am not able to do everything. Does that mean that I am disabled? (just trying to prove a point) Everyone one of us is disabled in some way. Not one of us can do everything that we want to do. Being 'disabled' isn't a bad thing. Being different is what makes this world an interesting and fun place to be!
Maybe instead of explaining to your daughter that 'that person' has something wrong...you can use what she is seeing/experiencing as a life lesson that can make the world a happier place for those 'disabled'?
My 'other half' and I also work with 'disabled' children and adults at DAMAR. We take the clients out all the time. Yes, they do at times get looks and are asked "what is wrong with you?" But, we've taught our clients that they are 'special' not wrong.
Even my now 10 yr old 'special' lil one sees that there are others who aren't able to do what she can, so she goes to offer help when she can. It is awesome to watch her! Seeing life thru the eyes of our children is an amazing gift...seeing life thru the eyes of a 'special' child/person is a gift from above!