There’s no built-in way to change the tag type for Hype elements. The way to insert
<h3>, etc. tags is to choose Edit > Edit Element’s Inner HTML for an existing element (like a Rectangle) and then add the HTML yourself.
It will use default browser styling, which means they will look different. If you want them to look the same you’ll need to add a
<style> tag to the Head HTML (accessible via the Document Inspector) that sets their style to be the same. You may also need to uncheck Protect from external styles also in the Document Inspector.