I would fall back on a variation of the standard line about differing abilities: "Sweety, she was not being rude. She simply didn't understand the way you were playing. Everyone is good at different things, and you are good at making words (or whatever thing she is amazed others can't do). She hasn't learned to do that yet, but she will. Would you like to make some words for me to read here or at home, or would you like to think of a different way that you could play with her that you could both do?"
I agree with you that 3 is early to have a full-on discussion of giftedness, but she is noticing the differences, so you have to validate her feelings yet help her figure out ways to play with normally-developing kids in a way that is still fun for her.
As for the other moms, you just have to ignore it/get used to it/get over it.
MUCH easier said than done!
