Running multiple php scripts into one php only, cron mail alert problem...

hi,
while separated they produce the usual mail alert and i can see the output...
if i write into the php script:

<?php 
system('php -f /var/www/vhosts/domain.com/httpdocs/folder/script1.php'); 
system('php -f /var/www/vhosts/domain.com/httpdocs/folder/script2.php'); 
system('php -f /var/www/vhosts/domain.com/httpdocs/folder/script3.php');  
?>

i don't receive anything by mail...
in your opinion what's the problem and how to force the output to the mail box? thanks