I have never used Plesk, but I see it is apache based.
Does it have a gui interface to administration?
Do you have all three IPs configured on the server? Does it take traffic on all three? If not, you have to set up some IP aliases on the (probable) single netword interface card.
As for apache, I usually do these things by hand, editing the main apache configuration file. It is usually called apache.conf or httpd.conf, and it'll be in /etc or /etc/apache or /etc/httpd (I'm a Debian user, not RH).
Apache will by default listen on all available IP, but you have to define what to put there. For that you use <VirtualHost> sections.
You have to fill in the necessary stuff in each section to define the three sites.
I hope this can help you a bit.