What can happen if my periods are 3months long or longer and I don't see a doctor?

Possibly trouble. If you mean you are bleeding for 3 months or longer, then you should see a doctor as soon as possible since the bleeding can signify different problems, even cancer. It can also cause severe anemia, which can cause other serious health problems. If you mean menstrual cycles, then this is not as serious. It may be due to not ovulating for various reasons. Your doctor can find out why.
Missing periods. It all depends on your age. In your teens it is common for periods to be very irregular for 6-12 months. If you are sexually active without birth control, of course, 3 months without periods may mean a surprise pregancy. In your 40s and 50s, this irregularity is common around the time of menopause, which can last for a year or more. Once periods stop for a year, and new periods need md evaluation.