A <sect3> element represents a third level section, always nested within a <sect2> . It may contain <sect4> sections, plus any other type of element supported by <sect1> with the exception of <sect2> elements.
See <sect1> for full details of how to use the various section tags.
Section three headings are of middling importance, and normally denote a sub topic. The title should not greatly break the flow of text.