I would not recommend it for a 3yo. Even a four year old would have trouble understanding the story line and for a 3yo would go over head! Can you see Madagascar 3 instead? I had doubts for my 10yo to see it until I saw the reviews. It is rated PG and does have scary scenes.
My 4 year old and 6 year old were fine in it, but it does have what they call the bogey man (pitch black). So for a sensitive child it could be quite scary with the dreams being turned to nightmares with scary horses and so on.
You've read the books and watched all the movies, now Harry Potter fans can visit the magical places from the wizarding world in person. Here's 10 of the best Harry Potter destinations every fan should see.