As s_ha_dum said, Widgets are **not meant** to be used like this.

If you want to change the behavior of all your h3 tags inside your posts, Wordpress has already article with "post" or "type-post" classes so you should modify your css like this :

``.post h3 {
    ***
}``

If you want your change to affect only a specific type of posts :

 1. Add a specific class ``mySpecificPostClass`` around the posts you want to change
 2. Modify your css style and use something like

``.mySpecificPostClass h3 {
    ***
}``

More detail [here][1].

  [1]: http://stackoverflow.com/questions/7468891/css-set-a-property-for-h2-to-h6-for-a-specific-class