zubair.am on "How to modify HTML of posts"

Hi,

On my blog I've close to 20 posts. I want to modify the <H3> tag that encapsulates headings inside my posts to add a class. For example, in my post I have a heading like <h3>My custom heading</h3>

I want to change the post to add a class to the H3 like <h3 class="H3Heading">My custom heading</h3>

I do not want to manually go and modify each and every post. What is the best way I can modify HTML of all the posts?

Any plugin that can help me?