WordPress Twenty Twelve Child Theme. Removing default widgetized areas

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The unregister_sidebar takes the ID, not the name:

<?php unregister_sidebar( $id ); ?>

You’ll need to do it like this:

<?php
function remove_some_widgets(){
    unregister_sidebar( 'sidebar-1' );
    unregister_sidebar( 'sidebar-2' );
    unregister_sidebar( 'sidebar-3' );
}
add_action( 'widgets_init', 'remove_some_widgets', 11 );
?>