How to change the default class for the widget title?

h2 and h3 are not html classes, they are html tags.

That said, does not exists a default html tag for the widgets, because the format used to output widgets title is set when the sidebar is registered (look at ‘before_title’ argument).

So how title is shown is theme related and sidebar related, does not depends on widgets.

An easy way to change the html tag your theme is using for widget titles, is to use the 'dynamic_sidebar_before' hook (fired just before the sidebar is shown) and global $wp_registered_sidebars variable to alter the ‘before_title’ argument on the sidebar being shown, thanks to the fact the hook pass the sidebar id:

add_action( 'dynamic_sidebar_before', 'widget_title_h2_h3' );

function widget_title_h2_h3( $sidebar_id ) {
 global $wp_registered_sidebars;
 if ( isset( $wp_registered_sidebars[$sidebar_id] ) ) {
    if ( isset($wp_registered_sidebars[$sidebar_id]['before_title']) ) {
      $now = $wp_registered_sidebars[$sidebar_id]['before_title'];
      $h3 = str_ireplace( '<h2', '<h3', $now );
      $wp_registered_sidebars[$sidebar_id]['before_title'] = $h3;
    }
 }
}

However this will change h2 to h3 in all your sidebars (at least all the sidebars where there is an h2 in the widget title).

To apply code to a specific sidebar, add a condition inside outer if that should appear like so:

if ( $sidebar_id === 'sidebar-id' && isset( $wp_registered_sidebars[$sidebar_id] ) ) {

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