Software for managing a dual gateway

With a proper hardware router with say two WAN links accessing the internet(two gateways), a network request coming through one link will be returned through that same link(I'm not interested in multiplexing and load balancing).

The equivalent of that in an operating system would be two gateways(one for each network interface), windows acts erratically with two gateways and I certainly can't set up how these gateways will be used. I've tried with a Kerio firewall, but no luck.

I need to set up a gateway PC with 3 network interfaces, two going to separate broadband links and one for the internal network, the gateway routes incoming traffic to mail and web servers, and the outgoing server traffic must be routed towards specific WAN links.

This problem has had me stumped for days!

Could you recommend me some session-persistent, load- balancing hardware routers?