Plesk will run fine on RedHat Enterprise Linux 3 (RHEL 3). CentOS is a distibution based on the RHEL 3 source RPMS and is almost totally compatible with RHEL 3. Plesk does however not recognise the OS identification string, to fix it do the following:
cp /etc/redhat-release /etc/redhat-release_original
echo "Red Hat Enterprise Linux ES release 3 (Taroon Update 4)" > /etc/redhat-release