please see the network map from http://i.stack.imgur.com/HqT7h.jpg

I want to connect to network3 from network1 but i want to connect by network2!
I have a PC which has Ubuntu 11.04 and it can connect to network3 by network2.
I have a client on network1 with default gateway=192.168.76.5(eth0 on PC).
so if the client want to connect to network 3,pc should route the traffic to network 2!
i want a help for configuring the PC:)
Thanks:)