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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.

Edit: if you want a plugin to do it, you will find a first answer here: 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