Is it possible to define a multi-step route in Linux?

My situation is that I use machines in three different, but connected, networks (we'll call them network 1, 2, & 3). My main machine is in network one and it's default route leads it out to the internet. I have a test machine in networks two and three (2 NICs), whose default route is network 3 leads out to the internet. I would like to route network traffic from my machine in network 1 out through network 3's default gateway instead of it's own. Is that possible? I have full control of at least one machine in all three networks but no access to any of the routers that control the default gateways.

many thanks.

man route . Concern yourself with 'metric' settings.