[Jellby]

You should learn about CSS, the "string-set" property, and other features specific to prince.

Without more information or testing, you could try:

h3 {
  string-set: chaptertitle content();
}

This means: every time an <h3> tag is found, define a string called "chaptertitle" (this is only the name I chose) with the content of the tag.

Then, the default page style in the plugin has:

  @top-center {
    font-size: 50%;
    font-style: italic;
    border-bottom: solid thin black;
    margin-bottom: 1mm;
    content: string(chaptertitle);
  }

which puts the value of "chaptertitle" (i.e., the content of the latest <h3> tag) on the center part of the header.