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//* One. Register the menu(s) using the init action hook NOT after_theme_setup

//* Two. Hook the menu into a theme hook location ( You could replace the 2nd 
//* step with the template code which displays the menu however this is not best 
//* practice when modifying Genesis and reserved for parent theme development)

//* Simply change the genesis_after_header hook to the correct hook location you want to display your menu.

//* You can also change the container class (nav-menu) to match the existing Genesis nav menu class or you 
//* would need to add a lot of CSS if you use a unique class.

function register_additional_genesis_menus() {

register_nav_menu( 'third-menu' ,
__( 'Third Navigation Menu' ));
}
add_action( 'init', 'register_additional_genesis_menus' );

add_action( 'genesis_after_header', 'add_third_nav' ); 

function add_third_nav() {

wp_nav_menu( array( 
'theme_location' => 'third-menu', 
'container_class' => 'genesis-nav-menu' ) );
}