theducks' suggestion will certainly work, and answers your question. However, if you are going to be fixing the headers anyway, might I recommend a different solution than using the blank lines and trying to set margins for a blank line??
Try using css to give your
headings the spacing you desire instead of the blank lines around them. You will have
exact control of the spacing rather than trying to guess with extra blank lines...they may not end up looking the way you expect on some readers/devices.
Code:
h3 {margin:1.2em auto; text-indent:0; text-align:center; font-family:serif}
<h3>Chapter 1</h3>
<p class="first">This is the first paragraph.</p>
<p>This is the second paragraph.</p>
<p>This is the third paragraph.</p>
etc.
You can certainly do that manually... although it might take more time than using a few simple regex find/replace.
Cheers!