Twenty Eleven Child Theme (Layouts)

functions.php

<?php

add_filter( 'twentyeleven_layouts', 'my_twentyeleven_layouts' );
add_filter( 'twentyeleven_layout_classes', 'my_twentyeleven_layout_classes', 10, 2 );

function my_twentyeleven_layouts( $layouts ) {
	$layouts['sidebar-content-sidebar'] = array(
		'value' => 'sidebar-content-sidebar',
		'label' => __( 'Two sidebars, content in the middle', 'twentyeleven' ),
		'thumbnail' => get_template_directory_uri() . '/inc/images/sidebar-content.png',
	);
	
	return $layouts;
}

function my_twentyeleven_layout_classes( $classes, $current_layout ) {
	if ( 'sidebar-content-sidebar' == $current_layout )
		return array( 'three-column', 'sidebar-content-sidebar' );
		
	return $classes;
}

style.css

.three-column {
    /* styles go here */
}