Hello,
This can be due to primary amenorrhea. It can be due to adrenogenital syndrome, chromosomal abnormalities, congenital adrenal hyperplasia, cushing’s disease, hypothyroidism, polycystic ovarian disease or tumors of the ovary or pituitary.
I suggest you to get it evaluated from a gynecologist to confirm the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Three could be a lot of different reasons, vitamin deficiency, hormone imbalances, even lack of a uterus. Without having tests done, it's really impossible to know.