Change footer widgets classes

change_footer_widgets_classes.php

add_filter('footer_one_widget_class', 'my_footer_widgets_class');
add_filter('footer_two_widget_class', 'my_footer_widgets_class');
add_filter('footer_three_widget_class', 'my_footer_widgets_class');
function my_footer_widgets_class( $class ){
    $widget_class = array(
        'footer_one_widget_class' => 'span3',
        'footer_two_widget_class' => 'span6',
        'footer_three_widget_class' => 'span3',
    );
    return str_replace( 'span4', $widget_class[current_filter()], $class);
}