3 months is a very long time to still be waiting to miscarry, you can get a horrible infection if you do not have the pregnancy tissue etc removed.
When i had my m/c I started spotting then 4 days later m/c completly with heavy dark bleeding and large clots, very painful, but natural.
I would definitly go to the doctors, I know you dont want a d and c but it might be your only option if your body hasnt done it naturally.
Good Luck hun.