I am 23. I stopped anticonception three years ago. I am still not pregnant. Could there be a serious problem?

Possibly. Certainly, your chances of becoming pregnant on your own are very low at this point. You certainly should see a specialist to be evaluated. Although it is possible there could something severe going more, more likely there is no serious problem. But there is a good chance you would benefit from some fertility treatment, depending on what the results show.
Infertile for 3 yrs. Less than 1% of normal couples would not be pregnant by 2yrs, so there must be some problem if not pregnant yet. You need testing and likely treatment. Fortunately, we are good at both and most people succeed but your age is the most hopeful fact. Testing is usually only a month. Most insurance covers tests but not all insurance covers treatment. Best wishes but get checked.
Yes. At 23, if it has taken you three years to conceive, something is wrong and you should begin to puruse evaluation.
Fertility testing. Fertility testing to confirm normal ovulation, sperm production and fallopian tube patency should be performed after trying for 1 year. Additional testing or treatment is based on these results. Testing and treatment is recommended earlier than 1 year if there are other problems like irregular periods or pelvic pain or the women is 35 years old or older.