logrotate is almost certainly already configured to run via cron. There will probably be a link to it in /etc/cron.daily.
Assuming your logrotate.conf is in /etc, you can check your configuration by running 'logrotate -d /etc/logrotate.conf'. That will print a summary of what your config does.
To do a test rotate, run 'logrotate -f /etc/logrotate.conf'.
I've made my own reporting utilities so I'm not sure what SARG does, but if you have lots of reports building up there's no reason not to use logrotate to manage them.
Using logrotate -d and logrotate -f produces the error:
error: /etc/logrotate.conf:36 duplicate log entry for /var/log/squid3/access.log
It sounds like you already had logrotate set up to rotate access.log. The configuration is probably in an include file in the /etc/logrotate.d/ directory. You just need to rationalize where you're going to keep the config - either in logrotate.conf or in the include file.