Search Multiple Fields

So I’ve added the following code to my functions.php which limits the search bar to ONLY look at last_name. But I’d like to expand it to include first_name and phone_number. How do I let the search bar cover those additional fields?

function kia_meta_search( $args ){

  // this $_GET is the name field of the custom input in search-author.php
    $search = ( isset($_GET['as']) ) ? sanitize_text_field($_GET['as']) : false ;

    if ( $search ){
        // if your shortcode has a 'role' parameter defined it will be maintained
        // unless you choose to unset the role parameter by uncommenting the following
        //  unset( $args['role'] );
        $args['meta_key'] = 'last_name';
        $args['meta_value'] = $search;
        $args['meta_compare'] = '=';

        // need to unset the original search args
        if( isset( $args['search'] ) ) unset($args['search']);
    }

    return $args;
}
add_filter('sul_user_query_args', 'kia_meta_search');