We have 3 x 3900 firewalls that all sit on separate segregated networks and all 3 have separate internet feeds.

Two of the networks have VPN access, Openvpn one one and Softether on the other (Both using NAT), I would like to be able to access the other networks via VPN. What would be the best way to do this, they are all housed in the same server room so physical connection is possible.

Is it as simple as putting them all on the same switch and doing a route add? We have had problems in the past connecting networks as the Windows DHCP and the Draytek DHCP bleeds over into the wrong network. I would like domain trusts between the 2 networks that run the VPN's