Heading 3, often abbreviated as H3, is a markup tag used in HTML to denote a subheading within a web page. It is typically used as a way to organize and structure content, with H1 being the main page title and subsequent headings indicating subtopics. H3 tags are smaller and less prominent than H1 and H2 tags, but still larger than regular paragraph text. They can help improve the accessibility and readability of web content for users and search engines, and also provide a logical hierarchy for web designers and developers to follow. H3 tags can be styled using CSS to fit the overall design of a web page.