Is it normal for a 13 year old girl to miss her period for 3 months?

I would say yes. If you are 13, you can't have been menstruating for long. It is very common for a girl who enters menarche (first period) to have very "irregular" periods for a year or two. If you are also of slender build, and very athletic, it is even more common. But if you have had a few periods, and you suddenly don't have one for six months, you must see a doctor. Now, all bets off on what is written above if you are sexually active, of course.