The three main causes of anemia are nutritional deficiencies, chronic diseases, and inherited or acquired blood disorders. Nutritional deficiencies, such as iron deficiency, vitamin B12 deficiency, and folate deficiency, can impair red blood cell production. Chronic diseases, including kidney disease, cancer, and autoimmune disorders, can affect the body's ability to produce or maintain healthy red blood cells. Inherited or acquired blood disorders, such as sickle cell disease or thalassemia, can also lead to anemia. It's important to identify the underlying cause of anemia through medical evaluation to determine appropriate treatment options.