Add one-third to sidebar contents.

// Only add the 'post_class' filters on the appropriate pages.
// The is_front_page() test was returining an incorrect value on non-front pages!!
add_action('get_header', 'mcl_add_post_class_filters');
function mcl_add_post_class_filters() {
    if (is_front_page()) {
        add_filter( 'post_class', 'mcl_front_page_sidebar_post_class' );
    }

    if (is_category()) {
        add_filter( 'post_class', 'be_archive_post_class' );
    }
}


// Change the front page sidebar items to be in 3 columns.
function mcl_front_page_sidebar_post_class( $classes ) {
    global $wp_query;
    if($wp_query->is_main_query() ) {
        return $classes;
    }

    if (!is_front_page()) {
        return $classes;
    }

    $classes[] = 'one-third';
    if( 0 == $wp_query->current_post || 0 == $wp_query->current_post % 3 )
        $classes[] = 'first';
    return $classes;
}


// Set the category archive posts to be in 2 columns.
/**
 * Archive Post Class
 * @since 1.0.0
 *
 * Breaks the posts into two columns
 * @link http://www.billerickson.net/code/grid-loop-using-post-class
 *
 * @param array $classes
 * @return array
 */
function be_archive_post_class( $classes ) {
    global $wp_query;
    if( ! $wp_query->is_main_query() ) {
        return $classes;
    }

    if (!is_category()) {
        return $classes;
    }

    $classes[] = 'one-half';
    if( 0 == $wp_query->current_post || 0 == $wp_query->current_post % 2 )
        $classes[] = 'first';
    return $classes;
}