value_1 = red blue; value_2 = gold silver;

merged_values = red blue gold silver;

// This function runs after your post is saved
function my_acf_save_post( $post_id ) {

    // Get new value of field 1 and remove ;
    $value1 = str_replace( ';', '', get_field( 'field1', $post_id ) );

    // Get new value of field 2 and remove ;
    $value2 = str_replace( ';', '', get_field( 'field2', $post_id ) );

    // Merged values with ; on the end
    $merge = $value1.' '.$value2.';';

    // Update field 3 with the new value which should be
    // value1 value2;
    update_field( 'field3', $merge, $post_id );
}
add_action('acf/save_post', 'my_acf_save_post', 20);