If you absolutely need the </h3> in the search to get the right matches, consider using a positive lookahead to avoid capturing/replacing it:
Code:
<a id="_idTextAnchor\d+"></a>(?=</h3>)
Then replace with nothing,
Or you can capture the /h3 and make sure it stays by using it in the replace expression:
Code:
<a id="_idTextAnchor\d+"></a></h3>
Then replace with: