Penis diameter of 3 cm is normal or not should be considered in conjunction with the patient’s age, whether the diameter when erect and the accuracy of the measurement. 1. Normal: under normal circumstances the average diameter of the adult male penis after erection is 2.5 ~ 4.0cm, about 1 ~ 2cm in the state of weakness, before puberty, the penis is short, but also for the development of the measurement of the diameter of the penis in the thickest part of the measurement and the error is small, at this time, the diameter of the penis 3cm belongs to the normal range. 2. abnormal: if children appear penis 3cm thickening performance is not normal, may be due to inflammation of the genitals, edema or fluid, in the exclusion of measurement error, should consult a doctor in a timely manner to identify the cause of the disease, so as not to miss the condition. Therefore, the adult male whether in a state of weakness or erectile state penis diameter of 3cm is normal, specific questions about the male reproductive organs, please consult a specialist in detail.