The mesiobuccal root of the upper molars usually presents anatomical complexities that limit the correct cleaning and shaping. This results in treatments with a high incidence of failure, which is why knowledge of the internal anatomy is essential. The presence of three main canals in the mesiobuccal root of maxillary molars is infrequent, which can make root canal treatment difficult. A clinical case of a maxillary left first molar with three mesiobuccal canals is presented here.