Hallo Sven,
find . -name "*.HTM" -exec rxr '<H3>(.*?)</H3>' '<p><strong>$1</strong></p>'
[...]
find . -name "*.HTM" -exec perl -pi -e 's!<H3>(.*?)</H3>!<p><strong>$1</strong></p>!sg'
Sorry, kleiner Fehler:
find . -name "*.HTM" -exec cat {} | rxr '<H3>(.*?)</H3>' '<p><strong>$1</strong></p>' ;
sowie
find . -name "*.HTM" -exec perl -pi -e 's!<H3>(.*?)</H3>!<p><strong>$1</strong></p>!sg' {} ;
Gruesse,
CK