I have two bottom molar fillings, I had one upper extraction of a middle molar and have a cavity. How bad are my teeth? I'm only 21.

Change habits. Starting today you can change your habits and prevent decay.Steps you can take to achieve this are: reduce sugars in your diet, eliminate sodas, energy drinks, bottled ice tea, etc. Even diet one hurt teeth. Excellent hygiene, brush and floss. Regular dental exam and cleaning, . Xylitol gum or mints. Fluoride rinse every night. These should help you.
Not Too Bad. However, you need to find out why are you getting all these cavities. Perhaps you need to lower your sugar intake, perhaps you need to be more dilligent in your oral care, or perhaps you need to see your dentist more regularly. Althuogh it could be much worse, it is time to take a look at what is contributing to your cavities, and take steps to reduce them. Good luck.