Do a search for <p class="calibre2"> to be sure that the
only usage is in the Chapter Headers
Code:
Search: <p (class="calibre2">.+)</p>
Replace: <h3 \1<</h3>
then do a single section Search and Replace.
Press F7 to see if the TOC has that correct
Abort the edit if wrong.
change to All HTML and find next, Replace All
F7 to check
Anything that is wrong in the files should be looked at and edited back to <p ... </p>,