An updated squid package has been released for CentOS-3

https://rhn.redhat.com/errata/RHSA-2004-591.html refers

Updated files are :-

squid-2.5.STABLE3-6.3E.2.i386.rpm

in updates/i386/RPMS/

These are available at http://mirror.centos.org/3.3/ and should be on all mirrors within a few hours.

To update to the latest version 'yum update squid' should be
sufficient.