mysql_insert_id NEED HELP

hi, i have three tables: customer username classification the user will enter my web site enter the relevant customer details (which are transfered to the database) (s)he will then enter their prefered username, password and a password hint (again all sent to the database). once they are registered they choose between three classification types. i have used the mysql_insert_id () to obtain the customer_id from the customer table and to insert it into the username table, however i also need to insert it into the classification table but all i keep getting is 0. can anyone advise, is it possible? liz lynch