Automate removal of a tag from all posts?

On a WordPress site I look after I recently discovered that strong tags are being added around all of the H3 tags in my articles. I haven’t figured out exactly what the cause of this was, but I suspect that it’s the WYSIWYG editor, which I have now disabled.

My problem is that now I have to remove all of the unnecessary strong tags that have been added because they are overriding my CSS. The site has about 200 articles with an average of 6-10 <h3> tags per article, so I’m going to have to remove up to 4,000 tags.

Can anyone suggest a plugin or other technique for automating the removal of the unnecessary H3s?

Thanks so much!