Static IP from some other ISP

if you add the third interface like you have above to build a DMZ, the routing is dead simple. quite literally all you need to do is enable IP Forwarding and it will forward packets between the connected interfaces.

either

sysctl -w net.ipv4.ip_forward=1

or

echo 1 > /proc/sys/net/ipv4/ip_forward

to enable it on the fly

or set

net.ipv4.ip_forward=1 in /etc/sysctl.conf

Then you would just need firewall rules to control access to the Lan computers from the DMZ, you would still utilize NAT from the Wan interface to the LAN interface, you would rely on Routing + access rules to control from the WAN to DMZ and the DMZ to LAN. The fact the DMZ and the LAN's default GW is the same box, you will not have to relay on static routes on the DMZ or LAN computers to access each other. Another option is to put 2 ethernet cards in each DMZ host, and connect them to the lan as well, but that isn't as clean as using the Router in the middle and access lists (iptables) to dictate what can talk to what.