Very easy, the answer is:
<h3><a id="c0_1" />THE TWINS</h3>
Calibre and Sigil were trying to fix it for you.
An epub's xhtml body can only contain block elements (div, p, blockquote, ul, , h1, etc). Text and anchors must be contained in a block element of some sort. Since your anchor here is merely acting as a target for an external reference, there's no need to wrap it around the text either, it just needs to sit next to it.