peter
I counted and you asked at least 3 questions, that's why I was answering.
Proxypass can be used for situations where you have a main gateway box that receives internal &/or external requests for valid domains configured on the gateway, but the web site for one (or more) of those domains is hosted on another box, either internal or external to your network.
eg you may host the mail for the domain but not the web site.
As you have oscommerce on a second test server, then you could give that server a different domain name to your main domain.
Then configure proxypass to send requests to the second server.
Alternatively configure a hostname (server3 or whatever name preferred) in the main server hostname & addresses panel, which points to the IP of server3, and call that hostname up when accessing oscommerce. The problem is what you have experienced already, oscommerce is confgured with the same URL as your main domain, so internally oscommerce keeps calling your main server.
Within the config files of oscommerce is where you set the domain name that oscommere is using (usually set at install/first config time), so change that too. It must be a setting that will resolve on your network of course, and as your server2 is acting as gateway/DNS for the local network, that URL needs to be known to server2 eg as a proxypass entry or a hostname.
I'm assuming here that when you setup server3 you told it to use server2 as the gateway & DNS server (in Configure this server during initial setup).