I want to create a plugin that adds specific class to all h3 tags inside posts, so it would not replace theme style but adds another one next to it. So for example

> h3 class="main_theme_class">Some title /h3

would become

> h3 class="main_theme_class my_class">Some title/h3

I need this to be done with a plugin, so I don't need to edit any file.