The three blood systems are the three types of blood cells in the blood, namely red blood cells, white blood cells and platelets. The number of all three blood cells in the blood must be maintained within a normal range, and any increase or decrease in the number is considered pathological. In clinical practice, there is often a decrease in the blood triad, which means that the number of leukocytes, red blood cells and platelets in the peripheral blood are significantly reduced. It can be seen in a variety of hematologic disorders, such as aplastic anemia, myelodysplastic syndrome, paroxysmal sleep hemoglobinuria, severe megaloblastic anemia, and severe iron deficiency anemia. The most effective differential diagnosis of trilineage reduction in peripheral blood, which can be seen in a variety of diseases, is bone marrow aspiration. Through bone marrow aspiration, a clear diagnosis of most trilineage reduction diseases can be made and clinical treatment can be guided.