The average period length for all women is 3 to 7 days so 3 days is within normal. Period length is different for every woman and might be different from cycle to cycle. But you are within normal. good luck
Early period length can be very irregular. Although your first period only lasted 3 days, it doesn't mean that future ones could not last for more time. When I started my period, there was no specific pattern. I would keep a pad with me if I were you because it could last longer. Hope this helped.