I am 18 years, a virgin and haven't had a period in 3 months. Should I be worried or go visit a doctor?

Assuming you have had no sexual contact (it is possible to get pregnant without full penetration) then I'd look to your lifestyle for the reason your periods have stopped. Have you been on a diet recently, or started intense training? Many dieters or people training for a marathon find their periods get light or stop.

Have you started any new medication recently? Many forms of birth control can affect periods, but others can too.

If it's none of those then I'd ask your doctor.