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].

Edit: if you want a plugin to do it, you will find an first answer here: http://wordpress.stackexchange.com/questions/21561/where-is-the-right-place-to-register-enqueue-scripts-styles.

For a tutorial on plugin creation, look here: https://codex.wordpress.org/Writing_a_Plugin

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