Title mobil smaller

If you want to replace it globlay and not on a specific h tag:

by the way – you definitly want to change the h-tag from h3 to h5
or do you only want to change the size in mobile devices ?
And do you like to have it definitly only for mobile devices or for smaller screens too?

The reason why i asked this is that on mobile devices the enfold adds to the html tag a class called: avia_mobile
so if you like to influence only the mobile version of a tag you can always do it via this class infront of the needed rule f.e.

.avia_mobile h3 {
 font-size: 24px !important
}

if you want the tags to be different you have to do it via child-theme functions.php
you can use this nice replace tag with tag function and an if clause –

function replace_tags_with_tags(){
if(wp_is_mobile()){
?>
<script>
  (function($) {       
      function replaceElementTag(targetSelector, newTagString) {
        $(targetSelector).each(function(){
          var newElem = $(newTagString, {html: $(this).html()});
          $.each(this.attributes, function() {
            newElem.attr(this.name, this.value);
          });
          $(this).replaceWith(newElem);
        });
      }
      replaceElementTag('h3', '<h5></h5>'); // you can add here more lines like this semicolon separated 
  }(jQuery)); 
</script>
<?php
}
}
add_action('wp_footer', 'replace_tags_with_tags');