According to the search results, the third leading cause of death in the United States is not consistent. However, some sources suggest that chronic obstructive pulmonary disease (COPD) is the third leading cause of death in the United States. Other sources suggest that car accidents, suicide, and homicide are the third leading causes of death in the United States. Accidents, including car accidents, are also listed as the fourth leading cause of death in the United States. It is important to note that the ranking of leading causes of death may vary depending on the source and the year.