If you're going to use tables, I believe you are required to use iptables to flag the packets you want to send to the correct table. I say this because I have a similar configuration (2 WAN, 1 LAN) at home and I use iptables to flag packets to the correct table for the WAN.
Are you bonding/balancing/failing over your external links? If so, you might as well pull up http://lartc.org/howto/
and memorize it. If you're not, you should be able to route all your traffic out the external link with a default route and set up normal routes like Dan said for each subnet and specify each NIC it goes to.
You might also need to disable rp_filter (not sure on this) and make sure you've enabled routing in sysctl.