How to insert certain code/text after all <h3>...</h3> elements?

How to insert certain code/text after all <h3>...</h3> elements?

How can I insert a certain text (or HTML code) after all <h3>.....</h3> elements?

A code like

  1. <h3>foobar headline</h3>
  2. nextline

should look like afterwards e.g.

  1. <h3>foobar headline</h3>
  2. insertedline
  3. nextline
resp

  1. <h3>foobar headline</h3>
  2. <p></p>
  3. nextline