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.
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