A: Dear Jess16310:
You can't teach 3 year olds about strangers, because the concept is over their heads. You can, however, teach a 3 year old to hold your hand and to stay with you in the store and not to wander off. You can impress upon a 3 year old that wandering off with anyone else is very, very bad, and that they absolutely must not do it under any circumstance.
This is just about as much information as a 3 year old can really absorb and it is just about as much information as they need. A child who is genuinely interested in the theory behind the parent's rules might inquire what the parent is afraid might hypothetically happen--then you can find a way to have an actual conversation with your child about the fact that occasionally bad people try to steal children from their parents for ransom or for some other nefarious purpose. But an authentic conversation like this requires powers of intellect and abstraction that no 3 year old possesses. A three year old might ask, "Why?" but the meaning of that question is simply a protest: "WHY do I have to do something I don't want to do!" The parent's answer here is to say enthusiastically and firmly, "Because it's a rule!"
Try that answer on a 3 year old. They usually accept it. They accept it because an intellectual capacity to comprehend theoretical reasons for precautions which are based on unlikely and dreadful possibilities is something that comes later.
Elizabeth Berger MD
Child Psychiatrist and author of "Raising Kids with Character"