The easiest way is to use .htaccess without touching the script code.

.htaccess

RewriteEngine On
RewriteRule ^cron/([^/]+)/([^/]+)/([^/]+)/?$ /path_to_script/cronjob.php?username=$1&password=$2&code=$3 [L,QSA]

result

* * * * * wget site.com/cron/test/test/1234