Hard lumps in the chest of a three-year-old girl may be caused by physiological factors of breast development, inflammatory infections, tumors and other pathological factors. Physiological factors do not require treatment, pathological factors need to be under the guidance of the physician medication. 1. Breast development: caused by physiological normal factors, the estrogen in the mother’s body will affect the child’s chest with hard lumps. When a girl is three years old, the nucleus of the breast may appear in the breast due to the hormone effect in the body, and there is generally no need to worry too much. 2. Inflammatory infections: If the child is infected with bacteria, there may be an infection of the thymus and mammary glands, and a hard lump in the chest may also appear. It may be due to improper squeezing, and can be treated with povidone-iodine disinfection, while appropriate amount of penicillin can be used to help anti-inflammatory under the guidance of physician. 3. Tumor: breast glands need molybdenum target photography, biopsy pathology to confirm the diagnosis, may be breast fibroadenoma, malignant tumors and so on. At the same time, hard lumps in the chest cavity may also be teratomas. If it severely compresses an organ, it needs to be surgically removed. Breast tumors require radiotherapy and anti-cancer recovery therapy under the guidance of a pediatrician. If a three-year-old girl has a hard lump, do not give your child self-medication, and it is recommended to seek medical attention as soon as possible to avoid further aggravation of the hard lump.