I can’t get my custom widget area to show on my WordPress site

Your have created three sidebars and that are primary_widget_area, secondary_widget_area, footer_widget_area

But in the sidebar, you call a different sidebar name sidebar-3; that’s why your sidebars are not displaying.

Try the following code in sidebar.php:

// To display primary_widget_area sidebar
<?php if ( is_active_sidebar( 'primary_widget_area' ) ) : ?>
    <?php dynamic_sidebar( 'primary_widget_area' ); ?>
<?php endif; ?>

// To display secondary_widget_areasidebar
<?php if ( is_active_sidebar( 'secondary_widget_area' ) ) : ?>
    <?php dynamic_sidebar( 'secondary_widget_area' ); ?>
<?php endif; ?>

// To display footer_widget_areasidebar
<?php if ( is_active_sidebar( 'footer_widget_area' ) ) : ?>
    <?php dynamic_sidebar( 'footer_widget_area' ); ?>
<?php endif; ?>

Thank you!