The age limit is there because most children before three still have the lanugo or "baby hair." This is similar to the hair on your body. Every style has some teasing in it which, if done on this young "hair", WILL NOT comb out. You'll just have to cut it out of the girls hair.
True, if you say she's three, they're not going to call you on it, but if she still has that hair or can't sit still and doesn't enjoy it because she freaks out, that's what you'll have to factor in.
It really all depends on the child, really. Just use your best judgement on her mood/maturity. If she goes in for the appointment and just doesn't want to have anything to do with it, you can always cancel the appointment.