Ok, this is my first post. I am a long time lurker and learner, but you have hit on a subject that I have expertise in. I am an educator by trade, kids are my business.
I agree with Massacre completely, though for a different reasons. At 3 years old, kids don't have the motor skills to properly perform karate forms. While it is cute to see, their effectiveness is very limited. Teachr her how to do summersualts, push ups, stand on one leg, balance on logs, jump rope, skip, hopscotch. Playing catch with a ball will help her with her hand/eye coordination. All of these things will help her large motor skills and help her when she is ready for Karate.
Five or six is a great time to start them. Six is nice because it gets them past the major changes of Kindergarten.
Both my daughters and wife study Isshyn Rue Karate. As Massacre stated, it is a no frills, but very competent form of the art. My oldest daughter is nine and this is her fourth year, she is now a junior black belt. She could hurt the average adult male. She won't recieve her full black belt until she turns 16, by that time, I won't have any fears in regards to her personal safety!!
Tom