HTML Heading 3, represented as `<h3>`, is a tag used in HTML to define a third-level heading in a webpage. It is part of a hierarchy of headings, which includes `<h1>` for the main title and `<h2>` for subheadings. The `<h3>` tag helps organize content, making it easier for users and search engines to understand the structure of the information presented. Using `<h3>` tags improves accessibility and SEO (Search Engine Optimization) by clearly indicating the importance of the text within the document. Properly structured headings, including `HTML`, `SEO`, and `accessibility`, enhance user experience and help search engines index the content more effectively.