I have an RF850 main router with two private networks behind it, and connect three IPSEC VPNs with RF820 routers from across the internet. How do I route traffic from one remote network to another through the main RF850?

If the two private networks behind a main site is part of a class B address and all the remote sites are on a different class B, you can setup one VPN tunnel for the class B subnet and then setup a static route in the RF850 to route the two private subnets.

If the above can not be done, you will need to create two separate VPN tunnels for the two private subnets for each of the remote location. Then setup static route on the RF850 to route the private networks.