What OS is the proxy server?
Most OSes should allow you to setup a "static route", which will turn the proxy server into a router. At the same time, you'll have to setup a new route on PC2, which points it to the PROXY server external IP in order to get to your internal subnet.
There are security implications to this, so do your reading before you decide to enable this.
This is all based on my assumptions...
You have some sort of ethernet based firewall/router connected to the Internet (via broadband modem)
PC1 and PC2 have NIC cards connected into the firewall/router
PC1 has a second NIC card, connected into a different hub/switch (not the firewall one)
PC3 (and others) each have a NIC card, and are connected into the hub/switch (not the firewall one)
The front end network (PC1, PC2, firewall/router) is subnet "A". The back end network (PC2, PC3, others) are configured on a subnet other than "A".