Hei I'm 15 and have not gotten my period for 3months for an unknown reason, I am still a virgin so I am not pregnant. What coul be wrong and can I start taking birth control now?

Just because you take birth control doesn't mean you won't or can't get pregnant. It can and does happen frequently. In fact statistically... 10 out of every 1,000 women taking it CORRECTLY will still get pregnant. I know some its happened to more than once. Also... it doesn't protect you from STD's. HPV (that can cause cervical cancer) and Herpes that has no cure... are among many that up to 28% of people your age are infected with. So look around the people your age... consider how many might be infected, many don't know and others simply lie about it to others.

You should be visiting a Gynecologist to be prescribed these... because they are not a one size fits all solution. Most can have significant side effects which are not the same between one female and another.

As for the other part of your question. You are 15, irregular periods are very common at your age. In fact even after you become an adult... your period will never be absolutely regular. Some are just more-so than others. Your best chance to get a valid answer for your problem is to see a gynecologist as it is dependent on exam results and medical history.